Started implementing Framebuffers and Renderbuffer

 * Specified and implemented GL.Objects.Renderbuffers
 * Moved GL.Low_Level.SizeI to GL.Types.Size because it's easier
   than using Natural in the interface and then use a cast every
   time.
 * Added new OpenGL 3/4 values to GL.Pixel_Data.Internal_Format
 * Improved GL.Objects.* a bit - the getters for the current
   object now return the same tagged type that was bound instead
   of always the base type (that may have been derived)
 * Did some smaller changes that I don't remember but are surely
   awesome.
 * Started specifying GL.Framebuffers