[Mesa-dev] Meta GenerateMipmap() support for array textures

[Kenneth Graunke]

I recently started looking at a suite of microbenchmarks, and discovered
that one of the tests called GenerateMipmaps repeatedly (sadly, probably
not intentionally).  Naturally, it used a 2DArray texture, which we didn't
support, so we hit the slow path.  Not only that, but it was half-float,
which meant we hit our -really- slow paths.

So, here's a small series that adds array texture support for GenerateMipmaps.
The first couple patches are just rearranging and tidying.

--Ken