[3.x] Allow negative scale in Particles and CPUParticles

[Calinou]

This can be used to make 2D particles look more 3D-y with thoughtful use of scale curve animation.

Negative scale is already allowed in master following the per-axis particle scale changes. Therefore, this PR is only relevant for 3.x. The zero scale workaround was taken from the master branch's ParticlesMaterial.

This closes #41587.

[akien-mga]

Seems fine to me, though too late for 3.5, but we can give it a try for 3.6-beta1.
Would be good to have a confirmation from @godotengine/rendering folks that this is safe enough.

[clayjohn]

I think this is probably safe enough. We still have issues with negative scaling (particularly as it scales normals and users don't expect that, see: #29317 and #47984) but for most situations it works fine.

[akien-mga]

Thanks!