Bugfixes:
- hddaudio now allocates it's decode buffer + AVCODEC_MAX_AUDIO_FRAME_SIZE
safety interval. (The former code expected all decoded audio frames to
be the same size which can hurt under some circumstances e.g. VBR files)
- writeffmpeg: some pointers where not initialized on start and after
deletion not set to null. Could segfault randomly on error conditions.
- drawseq: use startdisp and enddisp to decide, if a strip should be drawn.
(Just extend strips first or last frame beyond screen dimensions in
the previous version and watch the strip disappear)
df782b1122