[GtkGLExt] Problem choosing pixel format

[Jon Kinsey <jon_kinsey hotmail com>]

Hello,

I'm using the render to pixmap functionality (like the pixmap demo), it
fails on my windows machine at certain window/pixmap resolutions.

Looking into this I've found that there are two problems, at certain
resolutions there is only a valid pixel format with PFD_DRAW_TO_WINDOW
set (as well as PFD_DRAW_TO_BITMAP) and 32 bit pixmaps don't work.

A solution is to try a variety of (PIXELFORMATDESCRIPTOR) settings until
hopefully one that works is found.

I've hacked a version of gdkglpixmap-win32.c that works better locally,
not sure what the best solution is.

Jon