Use GLXFBConfig's depth when creating an XPixmap for a GLXPixmap

When creating a GLXPixmap we use an RGBA GLXFBConfig.  When creating the
XPixmap for that GLXPixmap we should use the depth from the XVisual
associated with that framebuffer configuration, as it might be 24 or 32.

Fixes #62.
Merge pull request #63 from mrobinson/issue-62

Use GLXFBConfig's depth when creating an XPixmap for a GLXPixmap