Add modified media-libs/mesa

Add gallium-nine media-libs/mesa, taken from FireBurn/Overlay and
modified to point at the current home of the gallium-nine fork of
Mesa-3D (currently maintained by okias, AKA David Heidelberger, and
located at the organizational repo of okias' sole proprietorship).

repo/branch, for reference:
git clone -b master git://github.com/iXit/Mesa-3D