Blend Modes Updates

This allows SFML.NET to work with the latest changes in SFML and my PR for CSFML. All is tested and seems to be working well. Just don't merge this until the CSFML PR is merged. :smile:

Also the second commit fixes an issue I noticed when using the latest SFML master. Since the fix to unbind textures during the clear() call, the OpenGL example was drawing an untextured cube.