sixel: destroy all sixels when font size is decreased

If changing the font size causes the cell size to decrease, either
horizontally or vertically (or both), then delete all sixels since the
grid space they allocated no longer is enough to hold the images.

terminal.c

@@ -1423,7 +1423,30 @@ term_font_size_adjust(struct terminal *term, double amount)
         return false;
     }
 
-    term_set_fonts(term, (struct fcft_font *[]){data[0].font_out, data[1].font_out, data[2].font_out, data[3].font_out});
+    const int old_cell_width = term->cell_width;
+    const int old_cell_height = term->cell_height;
+
+    if (!term_set_fonts(term, (struct fcft_font *[]){data[0].font_out, data[1].font_out, data[2].font_out, data[3].font_out}))
+        return false;
+
+    if (term->cell_width < old_cell_width ||
+        term->cell_height < old_cell_height)
+    {
+        /*
+         * The cell size has decreased.
+         *
+         * This means sixels, which we cannot resize, no longer fit
+         * into their "allocated" grid space.
+         *
+         * To be able to fit them, we would have to change the grid
+         * content. Inserting empty lines _might_ seem acceptable, but
+         * we'd also need to insert empty columns, which would break
+         * existing layout completely.
+         *
+         * So we delete them.
+         */
+        sixel_destroy_all(term);
+    }
     return true;
 }