temp/mesa-git: new aport

Some devices need bleeding edge drivers, for example the Hammerhead to have a non-broken Freedreno driver, the Pine devices to use newer versions of Lima (as it's currently an unstable driver still), etc. Every other device can use Alpine's drivers.

So to replace our current mess of Mesa packging, replace both our forked mesa and the Freedreno one with a general mesa-git package. This way we don't need to have it on version 9999, can use a bleeding edge version only for the devices that need it, and can enable new drivers in the future when needed before it's upstreamed to Alpine.

Devices requiring this driver rather than Alpine's should explicitly depend on mesa-git, mesa-git-glapi, mesa-git-gl and mesa-git-dri-<required driver> to not cause dependency conflicts. In theory a device can be switched back to a stable version of Mesa by removing -git from the Mesa entries in the dependency list, but I haven't tested this.

This MR basically makes !519 (closed) unnecessary.