VSE: Fix retiming unallowed strip type crashes

Currently, many retiming operators are able to operate on all selected
strips. However, if strips that do not support retiming are selected
(e.g. color strips), attempting to perform these operations will crash
Blender. The operators are only polled away if the active strip does
not support retiming -- this is not resilient enough.

This patch fixes the issue by checking each strip to make sure it
supports retiming. Some of the operators do not necessarily crash
without this fix (`retiming_show`, `retiming_key_add`), but still benefit
from exiting early if the current strip does not permit the functionality.

Pull Request: https://projects.blender.org/blender/blender/pulls/123975

source/blender/editors/space_sequencer/sequencer_retiming.cc

@@ -58,6 +58,9 @@ static void sequencer_retiming_data_show_selection(ListBase *seqbase)
     if ((seq->flag & SELECT) == 0) {
       continue;
     }
+    if (!SEQ_retiming_is_allowed(seq)) {
+      continue;
+    }
     seq->flag |= SEQ_SHOW_RETIMING;
   }
 }
@@ -68,6 +71,9 @@ static void sequencer_retiming_data_hide_selection(ListBase *seqbase)
     if ((seq->flag & SELECT) == 0) {
       continue;
     }
+    if (!SEQ_retiming_is_allowed(seq)) {
+      continue;
+    }
     seq->flag &= ~SEQ_SHOW_RETIMING;
   }
 }
@@ -222,6 +228,9 @@ static int retiming_key_add_from_selection(bContext *C,
   bool inserted = false;
 
   for (Sequence *seq : strips) {
+    if (!SEQ_retiming_is_allowed(seq)) {
+      continue;
+    }
     inserted |= retiming_key_add_new_for_seq(C, op, seq, timeline_frame);
   }
 
@@ -348,6 +357,7 @@ static bool freeze_frame_add_from_strip_selection(bContext *C,
 {
   Scene *scene = CTX_data_scene(C);
   blender::VectorSet<Sequence *> strips = ED_sequencer_selected_strips_from_context(C);
+  strips.remove_if([&](Sequence *seq) { return !SEQ_retiming_is_allowed(seq); });
   const int timeline_frame = BKE_scene_frame_get(scene);
   bool success = false;
 
@@ -646,6 +656,7 @@ static int strip_speed_set_exec(bContext *C, const wmOperator *op)
 {
   Scene *scene = CTX_data_scene(C);
   blender::VectorSet<Sequence *> strips = ED_sequencer_selected_strips_from_context(C);
+  strips.remove_if([&](Sequence *seq) { return !SEQ_retiming_is_allowed(seq); });
 
   for (Sequence *seq : strips) {
     SeqRetimingKey *key = ensure_left_and_right_keys(C, seq);