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.

CHANGELOG.md

@@ -38,6 +38,7 @@
   scrollback.
 * Mouse cursor from `hand2` to `left_ptr` when client is capturing the
   mouse.
+* Sixel images are now removed when the font size is **decreased**.
 
 
 ### Deprecated

sixel.c

@@ -66,6 +66,17 @@ sixel_destroy(struct sixel *sixel)
     sixel->data = NULL;
 }
 
+void
+sixel_destroy_all(struct terminal *term)
+{
+    tll_foreach(term->normal.sixel_images, it)
+        sixel_destroy(&it->item);
+    tll_foreach(term->alt.sixel_images, it)
+        sixel_destroy(&it->item);
+    tll_free(term->normal.sixel_images);
+    tll_free(term->alt.sixel_images);
+}
+
 static void
 sixel_erase(struct terminal *term, struct sixel *sixel)
 {

sixel.h

@@ -11,6 +11,7 @@ void sixel_put(struct terminal *term, uint8_t c);
 void sixel_unhook(struct terminal *term);
 
 void sixel_destroy(struct sixel *sixel);
+void sixel_destroy_all(struct terminal *term);
 
 /*
  * Deletes all sixels that are touched by the specified row(s). Used

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;
 }