limit the extraction to an rectangular region
a pointer to the newly allocated image surface. The caller must use cairo_surface_unmap_image() to destroy this image surface. This function always returns a valid pointer, but it will return a pointer to a "nil" surface if other is already in an error state or any other error occurs.
Returns an image surface that is the most efficient mechanism for modifying the backing store of the target surface. The region retrieved may be limited to the extents or NULL for the whole surface Note, the use of the original surface as a target or source whilst it is mapped is undefined. The result of mapping the surface multiple times is undefined. Calling cairo_surface_destroy() or cairo_surface_finish() on the resulting image surface results in undefined behavior. Since 1.12