Implement generalised image support

Support Kitty and SIXEL input and output with grid-resident image layers, history and copy-mode integration, text fallback rendering, and damage-rectangle redraws.

Consolidate the image work through 250f96b4b onto the redraw-damage-rectangles-new base, excluding the superseded redraw implementation and its historical merges. The original history is preserved in backup/4902-image-support-before-master-rebase-20261003.

format.c

@@ -3081,6 +3081,17 @@ format_cb_sixel_support(__unused struct format_tree *ft)
 #endif
 }
 
+/* Callback for image_support. */
+static void *
+format_cb_image_support(__unused struct format_tree *ft)
+{
+#ifdef ENABLE_IMAGES
+	return (xstrdup("1"));
+#else
+	return (xstrdup("0"));
+#endif
+}
+
 /* Callback for active_window_index. */
 static void *
 format_cb_active_window_index(struct format_tree *ft)
@@ -3753,6 +3764,9 @@ static const struct format_table_entry format_table[] = {
 	{ "host_short", FORMAT_TABLE_STRING,
 	  format_cb_host_short
 	},
+	{ "image_support", FORMAT_TABLE_STRING,
+	  format_cb_image_support
+	},
 	{ "insert_flag", FORMAT_TABLE_STRING,
 	  format_cb_insert_flag
 	},