Restore cursor when exiting alt mode.

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.

st.c

@@ -1379,6 +1379,8 @@ tswapscreen(void)
 	term.images_alt = im;
 	#endif // SIXEL_PATCH
 	term.mode ^= MODE_ALTSCREEN;
+	if (!IS_SET(MODE_ALTSCREEN))
+		xsetdefaultcursor();
 	tfulldirt();
 }