ffmpeg's libswscale is used to do RGB<->YUV conversions on movie reading
and writing. The "context" for the scale operation was being created
and destroyed for each movie clip / animation. Now, maintain a cache
of the scale contexts instead.
E.g. in Gold edit, it only ever needs two contexts (one for reading
source movie clips since they are all exactly the same resolution
and format; and one for rendering the resulting movie).
During playback, on some of the "slow" frames (camera cuts) this
saves 10-20ms (Windows, Ryzen 5950X). Rendering whole movie goes
from 390sec to 376sec.
