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Consolidate the image work through 250f96b4b onto the redraw-damage-rectangles-new base, excluding the superseded redraw implementation and its historical merges.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.

utf8.c

@@ -374,6 +374,10 @@ utf8_add_to_width_cache(const char *s)
 			wc_end = wc_start;
 		}
 
+		/*
+		 * wc_end may be WCHAR_MAX, so incrementing wc past it would
+		 * overflow (wc_start <= wc_end is already guaranteed above).
+		 */
 		wc = wc_start;
 		for (;;) {
 			utf8_insert_width_cache(wc, width);