Discern between virtual and abstract class bindings

* Previous "virtual" classes (which can't be instantiated) are not corretly named "abstract".
* Added a new "virtual" category for classes, they can't be instantiated from the editor, but can be inherited from script and extensions.
* Converted a large amount of classes from "abstract" to "virtual" where it makes sense.

Most classes that make sense have been converted. Missing:

* Physics servers
* VideoStream
* Script* classes.

which will go in a separate PR due to the complexity involved.

5 files changed, 83 insertions, 11 deletions

diff --git a/servers/audio/audio_effect.cpp b/servers/audio/audio_effect.cpp
index b9eca14a4c..f38d0adfb2 100644
--- a/servers/audio/audio_effect.cpp
+++ b/servers/audio/audio_effect.cpp
@@ -30,5 +30,36 @@
 
 #include "audio_effect.h"
 
+void AudioEffectInstance::process(const AudioFrame *p_src_frames, AudioFrame *p_dst_frames, int p_frame_count) {
+	if (GDVIRTUAL_REQUIRED_CALL(_process, p_src_frames, p_dst_frames, p_frame_count)) {
+		return;
+	}
+}
+bool AudioEffectInstance::process_silence() const {
+	bool ret;
+	if (GDVIRTUAL_CALL(_process_silence, ret)) {
+		return ret;
+	}
+	return false;
+}
+
+void AudioEffectInstance::_bind_methods() {
+	GDVIRTUAL_BIND(_process, "src_buffer", "dst_buffer", "frame_count");
+	GDVIRTUAL_BIND(_process_silence);
+}
+
+////
+
+Ref<AudioEffectInstance> AudioEffect::instantiate() {
+	Ref<AudioEffectInstance> ret;
+	if (GDVIRTUAL_REQUIRED_CALL(_instantiate, ret)) {
+		return ret;
+	}
+	return Ref<AudioEffectInstance>();
+}
+void AudioEffect::_bind_methods() {
+	GDVIRTUAL_BIND(_instantiate);
+}
+
 AudioEffect::AudioEffect() {
 }
diff --git a/servers/audio/audio_effect.h b/servers/audio/audio_effect.h
index 00a5d6c004..3a0578679d 100644
--- a/servers/audio/audio_effect.h
+++ b/servers/audio/audio_effect.h
@@ -33,20 +33,32 @@
 
 #include "core/io/resource.h"
 #include "core/math/audio_frame.h"
+#include "core/object/gdvirtual.gen.inc"
+#include "core/object/script_language.h"
+#include "core/variant/native_ptr.h"
 
 class AudioEffectInstance : public RefCounted {
 	GDCLASS(AudioEffectInstance, RefCounted);
 
+protected:
+	GDVIRTUAL3(_process, GDNativeConstPtr<AudioFrame>, GDNativePtr<AudioFrame>, int)
+	GDVIRTUAL0RC(bool, _process_silence)
+	static void _bind_methods();
+
 public:
-	virtual void process(const AudioFrame *p_src_frames, AudioFrame *p_dst_frames, int p_frame_count) = 0;
-	virtual bool process_silence() const { return false; }
+	virtual void process(const AudioFrame *p_src_frames, AudioFrame *p_dst_frames, int p_frame_count);
+	virtual bool process_silence() const;
 };
 
 class AudioEffect : public Resource {
 	GDCLASS(AudioEffect, Resource);
 
+protected:
+	GDVIRTUAL0R(Ref<AudioEffectInstance>, _instantiate)
+	static void _bind_methods();
+
 public:
-	virtual Ref<AudioEffectInstance> instantiate() = 0;
+	virtual Ref<AudioEffectInstance> instantiate();
 	AudioEffect();
 };
 
diff --git a/servers/audio/audio_stream.cpp b/servers/audio/audio_stream.cpp
index a710658bff..1ebd57fa7f 100644
--- a/servers/audio/audio_stream.cpp
+++ b/servers/audio/audio_stream.cpp
@@ -76,10 +76,10 @@ void AudioStreamPlayback::seek(float p_time) {
 
 int AudioStreamPlayback::mix(AudioFrame *p_buffer, float p_rate_scale, int p_frames) {
 	int ret;
-	if (GDVIRTUAL_CALL(_mix, p_buffer, p_rate_scale, p_frames, ret)) {
+	if (GDVIRTUAL_REQUIRED_CALL(_mix, p_buffer, p_rate_scale, p_frames, ret)) {
 		return ret;
 	}
-	WARN_PRINT_ONCE("AudioStreamPlayback::mix unimplemented!");
+
 	return 0;
 }
 
@@ -94,7 +94,7 @@ void AudioStreamPlayback::_bind_methods() {
 }
 //////////////////////////////
 
-void AudioStreamPlaybackResampled::_begin_resample() {
+void AudioStreamPlaybackResampled::begin_resample() {
 	//clear cubic interpolation history
 	internal_buffer[0] = AudioFrame(0.0, 0.0);
 	internal_buffer[1] = AudioFrame(0.0, 0.0);
@@ -105,6 +105,30 @@ void AudioStreamPlaybackResampled::_begin_resample() {
 	mix_offset = 0;
 }
 
+int AudioStreamPlaybackResampled::_mix_internal(AudioFrame *p_buffer, int p_frames) {
+	int ret;
+	if (GDVIRTUAL_REQUIRED_CALL(_mix_resampled, p_buffer, p_frames, ret)) {
+		return ret;
+	}
+
+	return 0;
+}
+float AudioStreamPlaybackResampled::get_stream_sampling_rate() {
+	float ret;
+	if (GDVIRTUAL_REQUIRED_CALL(_get_stream_sampling_rate, ret)) {
+		return ret;
+	}
+
+	return 0;
+}
+
+void AudioStreamPlaybackResampled::_bind_methods() {
+	ClassDB::bind_method(D_METHOD("begin_resample"), &AudioStreamPlaybackResampled::begin_resample);
+
+	GDVIRTUAL_BIND(_mix_resampled, "dst_buffer", "frame_count");
+	GDVIRTUAL_BIND(_get_stream_sampling_rate);
+}
+
 int AudioStreamPlaybackResampled::mix(AudioFrame *p_buffer, float p_rate_scale, int p_frames) {
 	float target_rate = AudioServer::get_singleton()->get_mix_rate();
 	float playback_speed_scale = AudioServer::get_singleton()->get_playback_speed_scale();
@@ -315,7 +339,7 @@ void AudioStreamPlaybackMicrophone::start(float p_from_pos) {
 
 	if (AudioDriver::get_singleton()->capture_start() == OK) {
 		active = true;
-		_begin_resample();
+		begin_resample();
 	}
 }
 
diff --git a/servers/audio/audio_stream.h b/servers/audio/audio_stream.h
index ce9bcabb9c..55031dec2c 100644
--- a/servers/audio/audio_stream.h
+++ b/servers/audio/audio_stream.h
@@ -81,10 +81,15 @@ class AudioStreamPlaybackResampled : public AudioStreamPlayback {
 	uint64_t mix_offset;
 
 protected:
-	void _begin_resample();
+	void begin_resample();
 	// Returns the number of frames that were mixed.
-	virtual int _mix_internal(AudioFrame *p_buffer, int p_frames) = 0;
-	virtual float get_stream_sampling_rate() = 0;
+	virtual int _mix_internal(AudioFrame *p_buffer, int p_frames);
+	virtual float get_stream_sampling_rate();
+
+	GDVIRTUAL2R(int, _mix_resampled, GDNativePtr<AudioFrame>, int)
+	GDVIRTUAL0RC(float, _get_stream_sampling_rate)
+
+	static void _bind_methods();
 
 public:
 	virtual int mix(AudioFrame *p_buffer, float p_rate_scale, int p_frames) override;
diff --git a/servers/audio/effects/audio_stream_generator.cpp b/servers/audio/effects/audio_stream_generator.cpp
index 6c917f3eb6..a3d615b925 100644
--- a/servers/audio/effects/audio_stream_generator.cpp
+++ b/servers/audio/effects/audio_stream_generator.cpp
@@ -169,7 +169,7 @@ float AudioStreamGeneratorPlayback::get_stream_sampling_rate() {
 
 void AudioStreamGeneratorPlayback::start(float p_from_pos) {
 	if (mixed == 0.0) {
-		_begin_resample();
+		begin_resample();
 	}
 	skips = 0;
 	active = true;