avfilter/vf_framerate: simplify filter

The framerate filter was quite convoluted with some filter_frame /
request_frame logic bugs. It seemed easier to rewrite the whole filter_frame /
request_frame part and also the frame interpolation ratio calculation part in
one step.

Notable changes:
- The filter now only stores 2 frames instead of 3
- filter_frame outputs all the frames it can to be able to handle consecutive
  filter_frame calls which previously caused early drops of buffered frames.
- because of this, request_frame is largely simplified and it only outputs
  frames on flush. Previously consecuitve request_frame calls could cause the
  filter to think it is in flush mode filling its buffer with the same frames
  causing a "ghost" effect on the output.
- PTS discontinuities are handled better
- frames with unknown PTS values are now dropped

Fixes ticket #4870.
Probably fixes ticket #5493.
Signed-off-by: Marton Balint <cus@passwd.hu>

tests/ref/fate/filter-framerate-12bit-up

@@ -62,3 +62,4 @@
 0,         56,         56,        1,   307200, 0x8cf55128
 0,         57,         57,        1,   307200, 0x4e740b42
 0,         58,         58,        1,   307200, 0x8e7e705c
+0,         59,         59,        1,   307200, 0xe73f29ef