UTF-8 combined character fixes.

Thai can have treble combinations (1 x width=1 then 2 x width=0) so bump the
UTF-8 cell data size to 9 and alter the code to allow this.

Also break off the combining code into a separate function, handle any further
combining beyond the buffer size by replacing the character with _s, and when
redrawing the UTF-8 character don't assume the first part has just been
printed, redraw the entire line.

tmux.h

@@ -516,7 +516,7 @@ struct grid_cell {
 } __packed;
 
 /* Grid cell UTF-8 data. Used instead of data in grid_cell for UTF-8 cells. */
-#define UTF8_SIZE 8
+#define UTF8_SIZE 9
 struct grid_utf8 {
 	u_char	width;
 	u_char	data[UTF8_SIZE];