mesa: overhaul the glGetTexImage code

mesa: overhaul the glGetTexImage code

1. Reorganize the error checking code.
2. Lay groundwork for getting sub images by passing image offset and
   dimensions to the error checking code.
3. Implement _mesa_GetnTexImageARB(), _mesa_GetTexImage() and
   _mesa_GetTextureImage() all in terms of get_texture_image().

v2: pass offset/width/height/depth arguments to the error checking
function, avoid using magic width/height/depth values.
v3: remove unused bufSize param to get_texture_image()

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