Remove Screen, SDL quick fixes, shader compiling functions

- Removed the Screen class; instead we use the module itself as the rendering
  entry point, and only ever have one screen which is created on call to
  vid_initialize.
- Switched back to core profile, and added C functions for compiling shaders
  into programs for later.
- Initialize SDL when the screen is created, and quit when the screen is
  destroyed.
- Added shaders for drawing and texturing to the resource zip.