New VSE cache implementation (!137926) uses the same eviction
logic for the source image ("raw") cache as for the final frame cache --
remove the frames furthest away from the playhead.
However, source image cache keys are source media frames, not timeline
frames, so this was not working correctly when strips are not at
timeline start.
So, for each cached entry record the strip-start-relative timeline
frame that the entry was created at. This takes care of things like
retiming, reversed frames etc. And also makes the cached frames
visualization more correct in presence of frame reversal etc.
Pull Request: https://projects.blender.org/blender/blender/pulls/139667
4de9801dd4