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.

extract.c

@@ -113,7 +113,7 @@ extract_one(const struct terminal *term, const struct row *row,
 {
     struct extraction_context *ctx = context;
 
-    if (cell->wc == CELL_MULT_COL_SPACER)
+    if (cell->wc >= CELL_SPACER)
         return true;
 
     if (ctx->last_row != NULL && row != ctx->last_row) {