term: rename CELL_MULT_COL_SPACER -> CELL_SPACER, and change its definition

Instead of using CELL_SPACER for *all* cells that previously used
CELL_MULT_COL_SPACER, include the remaining number of spacers
following, and including, itself. This is encoded by adding to the
CELL_SPACER value.

So, a double width character will now store the character itself in
the first cell (just like before), and CELL_SPACER+1 in the second
cell.

A three-cell character would store the character itself, then
CELL_SPACER+2, and finally CELL_SPACER+1.

In other words, the last spacer is always CELL_SPACER+1.

CELL_SPACER+0 is used when padding at the right margin. I.e. when
writing e.g. a double width character in the last column, we insert a
CELL_SPACER+0 pad character, and then write the double width character
in the first column on the next row.

vt.c

@@ -569,7 +569,7 @@ action_utf8_print(struct terminal *term, wchar_t wc)
         if (!term->grid->cursor.lcf)
             base_col--;
 
-        while (row->cells[base_col].wc == CELL_MULT_COL_SPACER && base_col > 0)
+        while (row->cells[base_col].wc >= CELL_SPACER && base_col > 0)
             base_col--;
 
         xassert(base_col >= 0 && base_col < term->cols);