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_defs.h

@@ -16,7 +16,9 @@
 // otherwise it must be a UTF-8 string of length maximum 4 (no NUL when n=4)
 
 typedef uint32_t schar_T;
-typedef int sattr_T;
+typedef int32_t sattr_T;
+// must be at least as big as the biggest of schar_T, sattr_T, col_T
+typedef int32_t sscratch_T;
 
 enum {
   kZIndexDefaultGrid = 0,