Do not create Pbuffers in advance (GH-27)

Instead, just record the dimensions and create during MakeCurrent, thus
following the route taken for other drawables and sidestepping the issue with
application creating a context with a Visual not 100% corresponding to an
FBConfig used to create the Pbuffer.
Allow glXQueryDrawable before a Pbuffer is made current
Try harder to find a somewhat matching Visual (GH-27, GH-87)