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.

ime.c

@@ -199,7 +199,7 @@ done(void *data, struct zwp_text_input_v3 *zwp_text_input_v3,
 
         for (int j = 1; j < width; j++) {
             cell = &seat->ime.preedit.cells[cell_idx + j];
-            cell->wc = CELL_MULT_COL_SPACER;
+            cell->wc = CELL_SPACER + width - j;
             cell->attrs = (struct attributes){.clean = 1};
         }
 
@@ -280,7 +280,7 @@ done(void *data, struct zwp_text_input_v3 *zwp_text_input_v3,
 
         /* Expand cursor end to end of glyph */
         while (cell_end > cell_begin && cell_end < cell_count &&
-               seat->ime.preedit.cells[cell_end].wc == CELL_MULT_COL_SPACER)
+               seat->ime.preedit.cells[cell_end].wc >= CELL_SPACER)
         {
             cell_end++;
         }