Rc/fix sprites

[jnterry]

Since no-one has been able to fix sprite sheets I cheated for the demo tomorrow - this uses loose textures and just switches between them.

Ideally this would be reverted after demo day, but it works so provided the performance hit isn't too great maybe this will be the final solution.

[AnthonyWharton]

Will this conflict with work in #164 before we go about merging this?

Paging @qqii

[qqii]

It may be fixed, but this would also be nice to have.

[qqii]

I'm actually a little worried about performance issues as the entire spritesheet won't be loaded at once. I think this is one to archive and look at if issues come back.