refactor(grid): implement rightleftcmd as a post-processing step

Previously, 'rightleftcmd' was implemented by having all code which
would affect msg_col or output screen cells be conditional on `cmdmsg_rl`.
This change removes all that and instead implements rightleft as a
mirroring post-processing step.

src/nvim/grid.h

@@ -29,6 +29,7 @@ EXTERN bool resizing_screen INIT( = 0);
 EXTERN schar_T *linebuf_char INIT( = NULL);
 EXTERN sattr_T *linebuf_attr INIT( = NULL);
 EXTERN colnr_T *linebuf_vcol INIT( = NULL);
+EXTERN char *linebuf_scratch INIT( = NULL);
 
 // Low-level functions to manipulate individual character cells on the
 // screen grid.