mesa: plumb offset/size parameters through GetTexSubImage code

mesa: plumb offset/size parameters through GetTexSubImage code

Needed for GL_ARB_get_texture_sub_image.  But at this point, the
offsets are always zero and the sizes match the whole texture image.

v2: Fixes, suggestions from Laura Ekstrand:
* Fix calls to ctx->Driver.UnmapTextureImage() to pass the correct
  slice value.
* Added comments and assertions to check zoffset+depth<=tex->Depth before
  the 'img' loops.
* Added a new zoffset==0 assert in get_tex_memcpy().

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>