OPENGL: Add another 16 bit mode.

I don't expect this to be commited, but I wanted to make sure there was a record of the issue. 

On my PowerBook G4, I was trying to run ResidualVM. This machine has an X server running in 16 bit mode rather than the normal 24 bit mode. The only OpenGL modes provided were 5650 rather than the 5551 expected by ResidualVM. This patch lets it run, but has a bad result, likely due to endian issues.