Fix (studio-reported) VSE prefetch process taking seconds to stop.

When VSE cache is invalidated (on most VSE edit operation e.g.), or
Blender quits, in some cases (complex and very long edits), the
prefetch job would block several seconds after being requested to stop.

This was because one codepath would keep looping over all frames without
checking for the `stop` flag.

source/blender/sequencer/intern/prefetch.cc

@@ -500,6 +500,10 @@ static void *seq_prefetch_frames(void *job)
     ListBase *channels = SEQ_channels_displayed_get(SEQ_editing_get(pfjob->scene_eval));
     if (seq_prefetch_must_skip_frame(pfjob, channels, seqbase)) {
       pfjob->num_frames_prefetched++;
+      /* Break instead of keep looping if the job should be terminated. */
+      if (!(pfjob->scene->ed->cache_flag & SEQ_CACHE_PREFETCH_ENABLE) || pfjob->stop) {
+        break;
+      }
       continue;
     }