Blender Git Loki

VSE: Improve thumbnail loading speed

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.