main: Add TEXTURE_CUBE_MAP support in CopyTextureSubImage3D.

main: Add TEXTURE_CUBE_MAP support in CopyTextureSubImage3D.

So it turns out that this doesn't actually fix any bugs or add any features,
stictly speaking. However, it does avoid a lot of kludginess.  Previously, if
you called

glCopyTextureSubImage3D(texcube, 0, 0, 0, zoffset = 3, ...

it would grab the texture image object for face = 0 in teximage.c instead of
the desired face = 3.  But Line 274 of brw_blorp_blit.cpp would correct for
this by updating the slice to 3.

This commit does the correct thing before calling any drivers,
which should make the functionality much more robust and uniform across all
drivers.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>