In cases of extremely short (single frame) input videos, the fps filter
can sometimes generate 0-frame output videos, resulting in the tpad
filter having no input (this breaks it, causing a busy loop).
Move the tpad filter to before the fps filter to solve this problem.
This isn't perfect, since the tpad filter doesn't work well on variable-
framerate video (it generates extremely high framerate video with a lot
of frames that will be discarded), but this only happens between the
tpad and fps filters, and only at the end of an input video (usually
right before a cut) so this seems acceptable.
Since the tpad and fps filter are in the same process, these duplicate
frames don't actually require copying any data (the frame is
reference-counted), and still process reasonably quickly.
Fixes#16407
46970162fb