Add texture atlas sprites

[cBournhonesque]

PR to be able to handle sprites that come from texture atlas.

My use-case is that all my image assets are stored in a texture atlas, so I need this change to be able to use the library.
I think that it would be interesting to let users have a particle sprite-sheet and the particles randomly use a sprite from the sprite sheet.

This PR is unpolished, I just opened it to see if it's a good idea for you.
If so, I would probably add an SpriteEnum to have 2 mutually exclusive options (Sprite or TextureAtlasSprite)

[abnormalbrain]

@cBournhonesque Thanks for the PR! I think this is an excellent idea and I agree with your approach.

I think something like a ParticleTexture enum with Sprite and TextureAtlas variants that contain the appropriate Handles would be good, in place of default_sprite.

I also like the idea of optionally randomizing the chosen Sprite on spawn. I'm not sure JitteredValue would be quite the right option for that, though. Particularly since it currently only operates on floats, and we'd need a usize. Also all valid options for the particle system system would have to exist in sequential indices in the texture atlas.

We can leave this part off for now and add it later too, but I think the best approach would be to do allow either defining something like RandomRange(Range<T>) or RandomChoice(Vec<T>) which allow ranges or sets of arbitrary numbers. What do you think?

[cBournhonesque]

Sounds great! I'll work on both changes!
Another thing that could be useful is z-axis rotation of the particles. I don't think i saw it in the options.

[abnormalbrain]

Yeah, there's no rotation currently, but that would also be great to add. For the sake of scope, that should probably be a different PR. I'll file an issue with some details.

[cBournhonesque]

@abnormalbrain I adjusted my PR to match your comment :)

I didn't add an example but have been using it in my game with TextureAtlasSprite

Probably another variant of RandomChoice would be an InOrderPicker where the values just cycle through the possible choices (Vec) in-order.

[abnormalbrain]

@cBournhonesque This looks great! Left one comment for discussion about copy vs clone, but I'm very happy with this and willing to merge as is.