Add animated textures to Particles 2D directed points

[afk-mario]

Currently if we want to use a different texture for different directed points the only way is to create a new Particles2D node with the same values as the original and with the different texture. It would save a lot of time and effort to be able to specify a different animated texture for each point.

A lesser important feature but a nice one would be to be able to flip the texture from the particles node.

It seems this was possible at some point but got removed in this commit:

godotengine/godot@95560e0

Not sure if it's possible for CPU particles but would be nice as well, currently we solved this by swapping the texture by code.