Fix T69924: Prefetch stops when moving playhead

Fix T70194: Prefetch freezes Blender in some cases

- Initialize depsgraph in isolated bmain struct.
- Fix prefetching range (forgot in initial prefetch commit).
- Fix crash - Add check if prefetch job is initialized and running.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D5899

1 files changed, 2 insertions, 1 deletions

diff --git a/source/blender/blenkernel/intern/seqcache.c b/source/blender/blenkernel/intern/seqcache.c
index d12710690df..ccb1869ee21 100644
--- a/source/blender/blenkernel/intern/seqcache.c
+++ b/source/blender/blenkernel/intern/seqcache.c
@@ -244,7 +244,8 @@ static SeqCacheKey *seq_cache_choose_key(Scene *scene, SeqCacheKey *lkey, SeqCac
    * We could use temp cache as a shield and later make it a non-temporary entry,
    * but it is not worth of increasing system complexity.
    */
-  if (scene->ed->cache_flag & SEQ_CACHE_PREFETCH_ENABLE) {
+  if (scene->ed->cache_flag & SEQ_CACHE_PREFETCH_ENABLE &&
+      BKE_sequencer_prefetch_job_is_running(scene)) {
     int pfjob_start, pfjob_end;
     BKE_sequencer_prefetch_get_time_range(scene, &pfjob_start, &pfjob_end);