Fix rounded-box-stencil and markup command \rounded-box
rounded-box-stencil was broken since:
commit 0772e38398972d6c2b4ba9e6f42e7725d973e08b
Author: Han-Wen Nienhuys <hanwenn@gmail.com>
Date: Sun Aug 1 11:15:02 2021 +0200
Stop passing color names to output backends
This patch removes the spurious call to ly:make-stencil that stopped rounded-box-stencil from working.
Also, rounded-box-stencil now returns the original stencil plus the surrounding rounded box as promised in the docstring.
Closes #6192 (closed)