Depth to texture

This commit 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