This patch changes the thumbnails' height used for image and movie
strips by choosing the proper size according to the VSE's text overlay
settings: i.e. thumbnails use the whole strip's height when no overlay
text is displayed; otherwise, some space is left for the overlay.
Reviewed By: ISS
Differential Revision: https://developer.blender.org/D13043
Split thumbnail job in 2 passes. First pass will render visible images
and second part renders set of "guaranteed" equally spaced images.
When viewing larger amount of strips, it is likely that only 1 or 2
images will be rendered in first pass, while in second pass it is up to
30 images.
This results (seemingly) in 3x better performance, but zooming before
set of guaranteed images is done will be slightly more inaccurate.
During rendering VSE cache is invalidated, so thumbnails would be
removed and thumbnail job would constantly restart.
Even if thumbnails would be preserved, resources should be dedicated
for rendering job.
Free strip `anim` data immediately after rendering. This doesn't affect
rendering performance, because each new loop would have to seek to start
of strip. Also strips are now freed anyway, but after rendering loop
ends.
With SF edit file, thumbnail rendering used around 60GB of memory.
Now it uses few hundreds MB (depends on movie file resolution, codec,
etc.)
Freeing of strips caused UI to be unresponsive for brief period. This
issue is not removed, but is more spread out so it is less noticable.
