This is possible because the lifetime of these structures coincide.
It has the advantage of allowing to remove AVHWFramesInternal
from the public header; given that AVHWFramesInternal.priv is no more,
most accesses to AVHWFramesInternal are no more; indeed, the only
field accessed of it outside of hwcontext.c is the internal frame pool,
making this commit very simple.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>