VSE: Indicate strips with missing media with red tint in the timeline

The sound strips actually reference bSound data block, and multiple
strips can reference the same one. Also when editing, it works
on bSound data, and you can't easily get to "which strips use it"
info.

So for sounds, cache media presence keyed by the bSound object itself,
not the sequencer strip.

@@ -256,1 +257,4 @@
   }
+
+  /* Clear sequencer media presence caches. */
+  blender::ed::seq::media_presence_free_all(bmain);

@@ -189,6 +189,7 @@ static SeqRenderData sequencer_thumbnail_context_init(const bContext *C)
   SEQ_render_new_render_data(bmain, depsgraph, scene, 0, 0, sseq->render_size, false, &context);
   context.view_id = BKE_scene_multiview_view_id_get(&scene->r, STEREO_LEFT_NAME);
   context.use_proxies = false;
+  context.never_show_missing_media = true;

@@ -112,7 +112,6 @@
  \
     .seq_prev_type = OB_SOLID, \
     .seq_rend_type = OB_SOLID, \
-    .seq_flag = 0, \

@@ -0,0 +53,4 @@
+      }
+      char filepath[FILE_MAX];
+      const char *basepath = get_seq_base_path(seq);
+      for (int i = 0; i < paths_count; i++, elem++) {

@@ -0,0 +76,4 @@
+  return false;
+}
+
+struct MediaPresence {

@@ -1042,3 +1060,3 @@
 
   if (ibuf == nullptr) {
-    return nullptr;
+    return create_missing_media_image(context, s_elem);

@@ -2475,0 +2476,4 @@
+  RNA_def_property_boolean_sdna(
+      prop, nullptr, "show_missing_media_flag", SEQ_EDIT_SHOW_MISSING_MEDIA);
+  RNA_def_property_ui_text(
+      prop, "Show Missing Media", "Render missing images/movies as solid color");