Strip without any processing can put single image into cache using
SEQ_CACHE_STORE_RAW and SEQ_CACHE_STORE_FINAL_OUT keys. This doesn't
increase memory usage however.

There was bug in memory usage counting. The code was removed and
function MEM_get_memory_in_use() is used instead.

If strip does perform processing, and this processing is very fast, we
could keep only SEQ_CACHE_STORE_RAW image or vice versa. But since
proxy raw images are not put into cache, this can cause problem when
prefetching is enabled, and no image is put into cache. Prefetching
always runs, untill cache is full.

We could check if heuristcs mentioned above will leave cache empty. in such case we would simply ensure that SEQ_CACHE_STORE_FINAL_OUT is stored.

However cost itself isn't enough to calculate real framerate because of UI overhead, whish is significant at higher framerates.
It is not easy measure UI overhead, in addition video codecs may behave differently during playback and scrubbing or random access.

I don't think I can implement this optimization well enough.

Caching raw images by default should be done after D9414 is implemented to conserve memory.