[mesa] only one mesa backend per image - multiples selected errors out on make world

On first glance I'm not sure it's intentional, but mesa backends are currently mutually exclusive (all of them set: PROVIDES:=libmesa).
Unfortunately that's not reflected in the menuconfig, where I can happily select e.g. both CONFIG_PACKAGE_libmesa-amd and CONFIG_PACKAGE_libmesa-intel.
However hitting make world then, the builds (not mesa's, though) fails with:

ERROR: unable to select packages:
  libmesa-amd-26.1.2-r1:
    conflicts: libmesa-intel-26.1.2-r1[libmesa=26.1.2-r1]
               libmesa-llvmpipe-26.1.2-r1[libmesa=26.1.2-r1]
    satisfies: world[libmesa-amd]
               libepoxy-1.5.10-r1[libmesa]
  libmesa-intel-26.1.2-r1:
    conflicts: libmesa-amd-26.1.2-r1[libmesa=26.1.2-r1]
               libmesa-llvmpipe-26.1.2-r1[libmesa=26.1.2-r1]
    satisfies: world[libmesa-intel]
               libepoxy-1.5.10-r1[libmesa]
  libmesa-llvmpipe-26.1.2-r1:
    conflicts: libmesa-amd-26.1.2-r1[libmesa=26.1.2-r1]
               libmesa-intel-26.1.2-r1[libmesa=26.1.2-r1]
    satisfies: world[libmesa-llvmpipe]
make[2]: *** [package/Makefile:100: package/install] Error 9

So right now, I can only have mesa with one backend in an image.

If this is intentional, we should probably mark the individual backends conflicting with each other.

Also: why? :) for mesa itself it should be totally fine to have multiple backends available at the same time.