Fix issues with retiming API

Ensure retiming data when operator is used, fix math for setting
retiming handle timeline frame.

source/blender/editors/space_sequencer/sequencer_retiming.cc

@@ -252,6 +252,8 @@ static int sequesequencer_retiming_handle_add_exec(bContext *C, wmOperator *op)
   const Editing *ed = SEQ_editing_get(scene);
   Sequence *seq = ed->act_seq;
 
+  SEQ_retiming_data_ensure(seq);
+
   float timeline_frame;
   if (RNA_struct_property_is_set(op->ptr, "timeline_frame")) {
     timeline_frame = RNA_int_get(op->ptr, "timeline_frame");

source/blender/makesrna/intern/rna_sequencer.c

@@ -358,7 +358,7 @@ static void rna_Sequence_retiming_handle_frame_set(PointerRNA *ptr, int value)
     return;
   }
 
-  const int offset = value - SEQ_time_start_frame_get(seq) + handle->strip_frame_index;
+  const int offset = value - (SEQ_time_start_frame_get(seq) + handle->strip_frame_index);
   SEQ_retiming_offset_handle(scene, seq, handle, offset);
   SEQ_relations_invalidate_cache_raw(scene, seq);
 }