Other than the general conversion:
* Made some slight aesthetic improvements.
** Removed gradients.
** Replaced stipples with transparency for hidden strips.
** Made strip borders less harsh.
** Removed stripes from offsets and made them brighter.
* Made only the visible parts of waveforms be drawn.
* Fixed a few drawing bugs.
** Background was not being drawn when buffer is NULL, and no
grease pencil is being drawn.
** Offset drawing ignored strip visibility.
Also, note that diagonal stripes for locked and error strips, are still
being drawn with the old api, as they await a new shader in order to
be converted.
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