mesa: Hide weirdness of 1D_ARRAY textures from Driver.CopyTexSubImage().

mesa: Hide weirdness of 1D_ARRAY textures from Driver.CopyTexSubImage().

Intel had brokenness here, and I'd like to continue moving Mesa toward
hiding 1D_ARRAY's ridiculousness inside of the core, like we did with
MapTextureImage.  Fixes copyteximage 1D_ARRAY on intel.

There's still an impedance mismatch in meta when falling back to read and
texsubimage, since texsubimage expects coordinates into 1D_ARRAY as
(width, slice, 0) instead of (width, 0, slice).

v2: Fix offset of scanline reads from the source. (Thanks Brian!), replace
    dd.h comment with Paul's text and replace early exit with an assert.

Reviewed-by: Brian Paul <brianp@vmware.com> (v1)
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v1)
Reviewed-by: Paul Berry <stereotype441@gmail.com> (v1)