Choose truly 32-bit framebuffer configurations when creating X11 surfaces

NVidia drivers have RGBA framebuffer configurations with 24-bit X
visuals associated with them. This means that Pixmaps created with these
visuals will be missing an alpha channel, leading to invisible textures
if that missing channel is interpreted as 0 alpha.

Fixes #65.
Merge pull request #66 from mrobinson/issue-65

Choose truly 32-bit framebuffer configurations when creating X11 surface...