glx: Always request an ARGB visual

glx: Always request an ARGB visual

When requesting the GLXFBConfig for creating the GLX context, we should
always request one that links to an ARGB visual instead of a plain RGB
one.

By using an ARGB visual we allow the ClutterStage:use-alpha property to
work as intended when running Clutter under a compositing manager.

The default behaviour of requesting an ARGB visual can be disabled by
using the:

  CLUTTER_DISABLE_ARGB_VISUAL

Environment variable.