Add SDL_GL functions for having a GL Context
GL: De-pointer-ize GLContext type
GL: remove redundant cgo command
GL: add comments
Fix comments
Merge pull request #11 from adam000/sdl-gl-begin-functions

Sdl-gl begin making some functions
Better rectangle and point types
Add Rectangle.IsEmpty method
Add Surface.Size method
Update showimage demo
Merge pull request #13 from zombiezen/rect

Make Rectangle type useful
Merge with upstream adam000/master
Merge remote-tracking branch 'origin/mainthread' into mainthread