Increase runtime requirement to Mesa 8.0

Adjustment to reality. KWin has had a dependency on Mesa 8.0 for quite
some time given that it is what basically all distributions ship.

It is better to clearly state what is required. For KWin Mesa 8.0 is a
more reliable dependency as all DRI drivers which do not support DRI2
have been removed.

Packagers have been informed about this intended change some weeks ago.

REVIEW: 106799