Update test-course for safety.

* Can no longer get stuck behind obstacle.
* Can no longer run off the edge of the tilemap.
Impl. `FixedBackground` as `sprite::Drawable`
Draw backdrop from `Game` behind all other sprites.
Move `screen width & height` constants to `Game`
Formatting
Move background from `Game` to the tilemap.
Add support for color-keying to graphics sprite loader
Add separate draw phase for backdrop.
Use color keying for sprites
Merge branch 'feature/backdrop'