Blender Git Loki

VSE: store raw images in cache

Previously raw images were not cached if image wasn't preprocessed.
This caused issue, that image had to be read from disk on every redraw.

Effect strips must be excluded, because this would cause problem with
invalidation. Effect strips can use preprocessing however. This is
mainly to allow usimg modifiers on them.

This change should follow rBf448ff2afe7a77, but I have wrongly assumed,
that it has been implemented already.

ref T80278