Particle emission points now have optional ordering modes

[QbieShay]

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This PR allows to leverage the full power of emission textures, giving optional full control to artists, including new point sort modes for when using points textures.

This PR only implements for 3.x and only for GPUParticles. It could be that I will finish it in the future and port it to 4.x, but anyone seeing this is encouraged to finish this work because I don't know when I will have time to finish it.

[QbieShay]

Thanks @Calinou ! Will update this PR today based on your feedback.

[QbieShay]

"Today" is once again a very long day.

It makes no sense to do this for 3.x anymore.

[akien-mga]

Closing as per above comment.