CPUParticles: Do randomness ratio computations in phase instead of time

The original shader code uses a phase (ratio from 0 to 1 for the particle
lifetime) for the randomness ratio computations, and this code was ported
over but converted to time computations.

The seeding/cycle logic was thus invalid, so we're going back to phase
for these computations, thus fixing the previous non-working time/emission
randomness property.

Part of #29692. Follow-up to #26859.

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