Depth to texture

This pull introduces saving depth to texture in render targets for OpenGL. I know that is not XNA feature but I think that is useful. I have tested this at my Windows 8 machine in desktop mode, but I don't simply have any other devices to test at. Test project that can be downloaded from here: http://t.zvb.cz/imperium/databaze/lidi/vodacek/TestDepthToTexture.rar