Deprecate redundant fract_delta property in GPUParticles

[Calinou]

This property was made redundant by the addition of interpolation in GPUParticles. It's now always enabled.

CPUParticles still features a fract_delta property for now. Should we remove it as well?

See discussion on the Godot Contributors Chat.

[lawnjelly]

CPUParticles still features a fract_delta property for now. Should we remove it as well?

Note that #52846 now uses the fract_delta for the timing calculations for "streaking" in interpolated CPUParticles (in 3.x but will likely be ported to 4.x). Streaking is using the fract_delta to back-calculate a "primed" position for restarting particle so they appear on an infinite timeline, rather than bunched up at update times only. I'm actually banditing the fract_delta property to also turn on and off streaking (rather than creating another specific property), I don't know what else fract_delta currently turns on / off.

I haven't yet investigated how the GPUParticles work or whether they could also do streaking (I'm sure it would be welcome there too if we can do it, although it doesn't have to be part of the interpolation PR). I don't know yet how the interpolation works in 4.x, whether it already can do this.

[MJacred]

@lawnjelly: fixed timestep interpolation for Godot 4 was implemented (#92391, #91818). A code search on Github says that fract_delta is unused.
Is there still sth. missing for Godot 4? If not, this property seems to remain redundant and could therefore be removed

[Calinou]

Rebased and tested again, it works as expected. This now preserves compatibility by deprecating the property and hiding it from the inspector instead of removing it entirely.

A code search on Github says that fract_delta is unused.

fractional_delta (and its resulting shader constant USE_FRACTIONAL_DELTA) were still used though. fract_delta is only used in the property names; all methods use fractional_delta instead.

I still think the property can be deprecated as I don't see any use cases to disabling it (unlike interpolation, which you may want to disable to sidestep rendering issues or have a retro look).