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.

terminal.h

@@ -48,7 +48,7 @@ static_assert(sizeof(struct attributes) == 8, "VT attribute struct too large");
 
 #define CELL_COMB_CHARS_LO    0x40000000ul
 #define CELL_COMB_CHARS_HI    0x400ffffful
-#define CELL_MULT_COL_SPACER  0x40100000ul
+#define CELL_SPACER           0x40100000ul
 
 struct cell {
     wchar_t wc;