Add improved coroutine macros.
Update coroutine example.
Move initialization to it's own procedure.
Use init-2d in all examples.
Use predefined tile layer in tilemap example.
Add short feature explanations and sample code to the README.
Fix README formatting.
Add window-title and set-window-title! procedures.
Update TODO.
Add rgba->gl-color procedure.

rgba->gl-color converts an RGBA color code in integer form into a list of color values for use with OpenGL calls.
For example, passing #xffffffff returns '(1.0 1.0 1.0 1.0).
Use rgba->gl-color in sprite module.
Mark task as complete in TODO.
Create animation type.
Add animation example.
Update TODO.
Change animation-state type to be immutable.
Fix typos and elaborate on coroutine example.
Update dependency information in README.
Update to guile-sdl 0.5.0 API.
Create gl module.