[X11] Fixed GLControl on nvidia binary drivers

Nvidia drivers fail in Glx.MakeCurrent() when using a 32bpp
visual on a window created with a 24bpp visual. Since we do
not know the actual visual until after the context is constructed,
the solution is to implicitly use 24bpp when 32bpp is requested.

The loss of the alpha channel does not have a user-visible effect,
since WinForms do not support translucent windows on X11.
[X11] Use the correct visual info for the context

This also fixes a potential NRE.