Fix threading conflict with movie cache line

It was possible that a render thread will be freeing cache while the
interface is iterating over cache items to build cache line.

Found while looking into T94738. It might be a fix, but I am unable
to reproduce the original issue, so can not know for sure whether
there is something else going or or not.

source/blender/blenkernel/intern/movieclip.c

@@ -1609,8 +1609,10 @@ void BKE_movieclip_get_cache_segments(MovieClip *clip,
   if (clip->cache) {
     int proxy = rendersize_to_proxy(user, clip->flag);
 
+    BLI_thread_lock(LOCK_MOVIECLIP);
     IMB_moviecache_get_cache_segments(
         clip->cache->moviecache, proxy, user->render_flag, r_totseg, r_points);
+    BLI_thread_unlock(LOCK_MOVIECLIP);
   }
 }
 

source/blender/editors/space_image/image_draw.c

@@ -541,7 +541,10 @@ void draw_image_cache(const bContext *C, ARegion *region)
     int num_segments = 0;
     int *points = NULL;
 
+    BLI_mutex_lock(image->runtime.cache_mutex);
     IMB_moviecache_get_cache_segments(image->cache, IMB_PROXY_NONE, 0, &num_segments, &points);
+    BLI_mutex_unlock(image->runtime.cache_mutex);
+
     ED_region_cache_draw_cached_segments(
         region, num_segments, points, sfra + sima->iuser.offset, efra + sima->iuser.offset);
   }