Simplify the implementation of the OpenGL classes...

Use a single macro.
Remove dependencies by moving the GL specific implementation into a .cpp file.
Remove the error checking macros introduced earlier.

Add a method to get the CUDA resource handle.
Call unregister() when registering a new graphics resource.
Add copyright notice
Get PixelUnpackBufferRT working...
Add gl/types.h

This file contains typedefs for the commonly used GL types.
Allows to use GL types in the API.
Add a method to display the color texture
Add gl/format.h

Helps dealing with internal OpenGL texture formats.
Add gl/types.h to CMakeLists.txt.
Add a new render target to render into FBOs
Add support for clearing the render targets in beginFrame()
Rename clear() to reset() and add a method which calls memset on the host device buffers
Add support for clearing the render targets in beginFrame() (continued...)