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.
Revert "Temporarily pretend that GLX_SGIX_pbuffer is supported (GH-27)"

This reverts commit 1a047f74fb024efdba145dc02332a268b066d694.
Do not try to match the exact MSAA factor

Fixes a regression on Torchlight (engine: OGRE), where it would select an
FBConfig with 2x FSAA, and then demanded a corresponding Visual to create a
window, but ddpy only had 4x and 8x FSAA.

Unfortunately, this Visual-FBConfig bridging is a bit fickle, since primus
should provide Visuals for all FBConfigs suitable for rendering into a window,
but restricting the set of exposed FBConfigs is undesirable, and neither is
virtualizing Visuals.
Update default nVidia path
Stop blaming Gentoo, simplify Ubuntu build instructions