charset,*: Refactor transstr()

src/nvim/mbyte.h

@@ -19,6 +19,13 @@
 #define MB_BYTE2LEN(b)         utf8len_tab[b]
 #define MB_BYTE2LEN_CHECK(b)   (((b) < 0 || (b) > 255) ? 1 : utf8len_tab[b])
 
+/// Maximum value for 'maxcombine'
+///
+/// At most that number of composing characters may be attached to the leading
+/// character by various `utfc_*` functions. Note that some functions do not
+/// have this limit.
+enum { MAX_MCO = 6 };
+
 // max length of an unicode char
 #define MB_MAXCHAR     6