mesa: handle the pbo case for save_Bitmap

mesa: handle the pbo case for save_Bitmap

Wrap _mesa_unpack_bitmap to handle the case that data is stored in pixel
buffer object.

This would make calling Bitmap with data stored in PBO by display list work.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>