st/mesa: fix blit-based GetTexImage for 1D array textures

st/mesa: fix blit-based GetTexImage for 1D array textures

This is not easy to hit, because we have 3 code paths now
(tried in this order):
- memcpy-based (skips the blit) -> _mesa_tex_getimage
- blit-based
- slow pixel packing -> _mesa_tex_getimage

The main difference later in the code is the parameters of
_mesa_image_address3d.

Reviewed-by: Brian Paul <brianp@vmware.com>