These filters do not directly know whether the API they are using will
support dynamic frame pools, so this is somewhat tricky. If the user
sets extra_hw_frames, we assume that they are aware of the problem and
set a fixed size based on that. If not, most cases use dynamic sizing
just like they did previously. The hardware-reverse-mapping case for
hwmap previously had a large fixed size (64) here, primarily as a hack
for QSV use - this is removed and extra_hw_frames will need to be set
for QSV to work since it requires fixed-size pools (as the other cases
do, and which didn't work before).
Also adds a new flag to mark filters which are aware of hwframes and
will perform this task themselves, and marks all appropriate filters
with this flag.
This is required to allow software-mapped hardware frames to work,
because we need to have the frames context available for any later
mapping operation in the filter graph.
The output from the filter graph should only propagate further to an
encoder if the hardware format actually matches the visible format
(mapped frames are valid here and have an hw_frames_ctx, but this
should not be given to the encoder as its hardware context).
