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
140ff12eae