Improved waveform drawing in the sequencer.
* Drawing the waveform of a sequencer strip is now independent from whether the sound is cached or not.
* Improved drawing of the waveform in the sequencer (especially speed!).
* Making it possible to vertically zoom more in the sequencer to better see the waveform for lipsync.
* Fixed a bug which crashed blender on loading a sound file via ffmpeg.
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