This is an update of https://developer.blender.org/D4426, a patch to add previews to movie strips in the sequencer.
This diff is just a port of @mathers' work to make the feature work on the latest master, but it carries the todos left from the original
{F7776246}
I don't know blender and the sequencer's codebase well, plus the sequencer's sources changed since Richard commented on the original issue, so I'd appreciate guidance on how to draw the thumbnails without leaking memory now.