Improve video scrubbing with FFmpeg backend

Video scrubbing (scrolling forward or backward through a video file, e.g. by dragging a position slider) is very laggy with our FFmpeg backend compared to darwin backend on macOS. The performance is better with low res video files. When scrubbing too fast, the view fails to update completely, regardless of the video resolution.

On Linux, FFmpeg is less laggy than gstreamer, but is still laggy and fails when scrubbing too fast.

Not tested on other platforms.