As discussed on IRC, we need a 'mesa-kit' with branches for at least 17.3, 18.0, 18.1, and 18.2-dev (bleeding edge). We should probably provide branches for 17.[012], and possibly 13.0 as well, either individually or collectively (mesa-old?).

At the minimum, this should include media-libs/mesa and app-eselect/eselect-mesa, and probably x11-lib/libdrm as well. Other packages which have tight versioning binding to mesa should be considered for this kit or to depend on this kit (once such a feature is available).

This allows the mesa version to be set using the profile in each branch of the kit and no masking/unmasking is required.