Takes a frame associated with a hardware context as input and maps it
to something else (another hardware frame or normal memory) for other
processing.  If the frame to map was originally in the target format
(but mapped to something else), the original frame is output.

Also supports mapping backwards, where only the output has a hardware
context.  The link immediately before will be supplied with mapped
hardware frames which it can write directly into, and this filter
then unmaps them back to the actual hardware frames.