Enable direct rendering for all Mesa drivers

As KWin requires at least Mesa 7.10 for OpenGL we can be sure
that all Mesa drivers support DRI2 and can enable direct rendering
for it.

Parsing the version string to identify if it is a mesa driver.
This can cause breakage again in future versions of Mesa.
Unfortunately version, vendor and renderer are the only information
queryable just with OpenGL API.

FIXED-IN: 4.7.0