Fix #124584: VSE thumbnail transparency for muted strips is wrongly stored in cache

Make sure the code that alters the VSE thumbnail to add transparency
(for disabled strips) works on a copy of the image, so that the extra
transparency does not get "stored" into the thumbnail cache.

Pull Request: https://projects.blender.org/blender/blender/pulls/124689

source/blender/editors/space_sequencer/sequencer_thumbnails.cc

@@ -622,6 +622,11 @@ void draw_seq_strip_thumbnail(View2D *v2d,
     /* Transparency on mute. */
     bool muted = channels ? SEQ_render_is_muted(channels, seq) : false;
     if (muted) {
+      /* Work on a copy of the thumbnail image, so that transparency
+       * is not stored into the thumbnail cache. */
+      ImBuf *copy = IMB_dupImBuf(ibuf);
+      IMB_freeImBuf(ibuf);
+      ibuf = copy;
       make_ibuf_semitransparent(ibuf);
     }