Allow having more animations with the same sprite

[shikaan]

We are currently running with the assumption that animations animate all the frames in a sprite.

There are cases where the sprite does not match an animation one to one, but it's rather a collection of frames which needs to be arranged in a certain way to put together an animation.

For example,

Here the first and the fourth frames represent the walk down animation. It's composed of 4 frames:

1. the fourth frame
2. the first frame
3. the fourth frame horizontally flipped
4. the first frame again

It would be nice to have animations that account for this scenario. Something like

auto animation = Animation(
  "sprite",     // Sprite label
  {3, 0, 3, 0}, // Frame index to be chosen
  {{}, {}, Transform{.flipX = true}, {}}
)