[Cogl] Another version of stereo patches

[otaylor at redhat.com]

Here's another attempt at the stereo patches. Changes are:

 - Removal of fallback-code - if no stereo fbconfig is available,
   display setup just fails.
 - Fixes for various leftovers that were pointed out
 - Minor rebasing on top of the EGL_KHR_surfaceless_context
   patch (added an assertion to catch if things get reordered
   in the future and glDrawBuffer() calls could interfere.)

I didn't change the CoglTexturePixmapX11 patch, since the suggested
change doesn't work - see my mail.

I went ahead and pushed the two OK'ed fixes to the texture-pixmap
example to cogl-1.18 and removed them from the series.