Restrict matching of Visuals for FBConfigs (GH-27)
Restrict matching of Visuals for FBConfigs (GH-27)

Return NULL visual if attributes do not match, otherwise if an application
first creates a Pbuffer with an FBConfig, but then decides to use a Visual
to create a context, it might get a BadMatch error due to different alpha.

amonakov/primus,amonakov/primus