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