If the mouse cursor is changed to a bar or an underline then st will use that
when the terminal is first opened. When an application that changes the cursor
via escape sequences is executed, e.g. vim which uses a block cursor by default,
then that cursor will remain after exiting the program.
This change sets the cursor back to default when exiting alt mode.
When an OCS sequence was used to change the bg color, the borders where
dirty. This simple patch just clears the window before the redraw of the
terminal when the bg color has been changed. This is apparently enough
and seams to be very smooth. There was a TODO comment for it on the st.c
file, which I removed.