As part of PR #118396 the "generator" effect strips (color, text,
adjustment) also started to put their "raw" images into the cache.
However, seq_cache_timeline_frame_to_frame_index that has optimization
that tries to use only one cache entry based on how many frames are
present in the strip was not expecting to see effect strips, ever.
As a result, something like adjustment layer would effectively just
cache a single frame, since it has no "source frames" to speak of.
Fixed that by only doing this optimization for non-effect strips.
6826efdb02