meta: Support GenerateMipmaps on 1DArray textures.

meta: Support GenerateMipmaps on 1DArray textures.

I don't know how many people care about this case, but it's easy enough
to do, so we may as well.  The tricky part is that for some reason Mesa
stores the number of array slices in Height, not Depth.

I thought the easiest way to handle that here was to make Height = 1
(the actual height), and srcDepth = srcImage->Height.  This requires
some munging when calling _mesa_prepare_mipmap_level, so I created a
wrapper that sorts it out for us.

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>