lavfi/avf_showspectrum: set output frame rate.
lavfi/avf_showspectrum: fix output pts computation.
lavfi/avf_showspectrum: do not push the frame at EOF.

It is always identical to the last pushed frame.
The samples in the last incomplete window were ignored,
this is unchanged.
Possible enhancement: pad the last incomplete window with
silence.
lavfi/avf_showspectrum: use automatic framing.

The framework can ensure that each input frame has exactly
the correct number of samples, except the last one.
lavfi/avf_showspectrum: add full frame sliding mode.
lavfi/avf_showspectrum: check RDFT context init.

Fix a segfault with large window size.
Merge remote-tracking branch 'cigaes/master'

* cigaes/master:
  lavfi/avf_showspectrum: check RDFT context init.
  lavfi/avf_showspectrum: add full frame sliding mode.
  lavfi/avf_showspectrum: use automatic framing.
  lavfi/avf_showspectrum: do not push the frame at EOF.
  lavfi/avf_showspectrum: fix output pts computation.
  lavfi/avf_showspectrum: set output frame rate.

Merged-by: Michael Niedermayer <michaelni@gmx.at>