meta: Support GenerateMipmaps on 2DArray textures.

meta: Support GenerateMipmaps on 2DArray textures.

This is largely a matter of looping over the number of slices/layers,
and not minifying depth (presumably that code exists for the unfinished
3D texture support).

Normally, I would have made the loop over array slices the outermost
loop.  I suspect that would make it trickier to support 3D textures
someday, though, so I didn't.  The advantage is that we would only have
one BufferData call per slice, rather than one per miplevel and slice.

However, a GenerateMipmaps microbenchmark indicates that either way is
basically just as fast.  So I'm not sure it's worth bothering.

Improves performance in a GenerateMipmaps microbenchmark by nearly 5x.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>