VSE: Add Box Roundness option to text strips

The background box for VSE text strips can have rounded corners now.

Actual rounded shape is a superellipse with 2.1 exponent; this is
very close to a circle section but feels a bit nicer with more
continuity between the flat part and the rounded part of the box.

At very large rounding radius this is not very fast; optimization
for that case will come in a separate commit.

Pull Request: https://projects.blender.org/blender/blender/pulls/129665

scripts/startup/bl_ui/space_sequencer.py

@@ -1725,6 +1725,11 @@ class SEQUENCER_PT_effect_text_style(SequencerButtonsPanel, Panel):
         sub.prop(strip, "box_margin")
         sub.active = strip.use_box and (not strip.mute)
 
+        row = layout.row(align=True, heading="Box Roundness")
+        sub = row.row(align=True)
+        sub.prop(strip, "box_roundness")
+        sub.active = strip.use_box and (not strip.mute)
+
 
 class SEQUENCER_PT_source(SequencerButtonsPanel, Panel):
     bl_label = "Source"