UI.Sprite: Implement nested blitting of textures.

Unfortunately, it's a very ugly implementation which is inefficient as
hell and needs a LOT of memory. Right now, my goal was to just have a
working version of it in order to experiment with it. So in the end, the
implementation will change anyway.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Add a few sprites for experimenting with nesting.

In the end we're going to support revos and LED sticks, so this are a
few sprites that we're going to use. Eventually we're going to add wires
for nested sprites, so we might end up creating separate textures in GPU
memory.

But so far and as I mentioned in the previous commit, it's for
experimentation purposes.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
default.nix: Remove Nix expression for sdl2.

It's now in upstream <nixpkgs> and thus no longer needed on our end.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>