Add a way of applying rotation over time to particles.

[Giufin]

Hello,

I was trying to make particles that spin in a loop at some point in their lifetime. However, unless the center of the loop is also the spawn point it is currently not possible.

This can be easily accommodated by adding a new acceleration curve that would apply rotation over time.
A pass of such acceleration would run arctan on the x and y velocity, increment it by the rotation value on given point (can be negative) and construct x and y velocity back with tan.

It's easy to do, but I lack the familiarity with the codebase to approach this myself.

Thank you

[Calinou]

It's easy to do, but I lack the familiarity with the codebase to approach this myself.

The ParticleProcessMaterial code (used by both GPUParticles3D and GPUParticles2D) is here: https://github.com/godotengine/godot/blob/bdf625bd54958c737fa6b7213b07581cc91059ad/scene/resources/particle_process_material.cpp

It procedurally generates a particle shader using the properties defined by the user. This toggles sections of the generated code as needed.

For CPUParticles3D and CPUParticles2D, all animation is done in C++ instead.

[passivestar]

Related/duplicate #3951