Blender Git Loki

VSE: cache performance optimization

Map frame for cached raw images to strip input media frame range. This
means that static images or extended frame range of movies will only
generate one cache entry.

timeline_frame is stored in cache key as a reference - on what frame
was this entry created, so we don't have to reverse lookup frame range.

Since each media frame corresponds to one cache frame with same frame
index key, there is no need to invalidate raw cache when changing time
remapping properties like use_reverse_frames or strobe

No changes are needed for disk cache, since invalidating raw entry
assumes all data will be invalidated.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D9462