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Consolidate the image work through 250f96b4b onto the redraw-damage-rectangles-new base, excluding the superseded redraw implementation and its historical merges.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

tty-features.c

@@ -236,9 +236,11 @@ static const struct tty_feature tty_feature_strikethrough = {
 	0
 };
 
+#define TTY_FEATURE_SYNC "Sync=\\E[?2026%?%p1%{1}%-%tl%eh%;"
+
 /* Terminal supports synchronized updates. */
 static const char *const tty_feature_sync_capabilities[] = {
-	"Sync=\\E[?2026%?%p1%{1}%-%tl%eh%;",
+	TTY_FEATURE_SYNC,
 	NULL
 };
 static const struct tty_feature tty_feature_sync = {
@@ -369,6 +371,50 @@ static const struct tty_feature tty_feature_sixel = {
 	TERM_SIXEL
 };
 
+/* Terminal has Kitty graphics protocol Unicode placeholder capability. */
+static const char *const tty_feature_kitty_capabilities[] = {
+	"Kty",
+	TTY_FEATURE_SYNC,
+	NULL
+};
+static const struct tty_feature tty_feature_kitty = {
+	"kitty",
+	tty_feature_kitty_capabilities,
+	TERM_KITTY
+};
+
+#ifdef ENABLE_IMAGES
+static const char *const tty_feature_image_capabilities[] = {
+	NULL
+};
+static const struct tty_feature tty_feature_image_quadrants = {
+	"image-quadrants",
+	tty_feature_image_capabilities,
+	TERM_IMAGE_QUADRANTS
+};
+static const struct tty_feature tty_feature_image_sextants = {
+	"image-sextants",
+	tty_feature_image_capabilities,
+	TERM_IMAGE_SEXTANTS
+};
+
+/*
+ * Terminal moves SIXEL or Kitty image content along with the rest of a
+ * scrolling region, rather than needing it redrawn after every scroll.
+ * There is no way to ask a terminal this, and it does not correlate with
+ * DECSLRM/margins support - confirmed by direct testing that mintty
+ * scrolls text within a margin-bounded region correctly but drops sixel
+ * content placed there, while WezTerm and Windows Terminal move it
+ * correctly - so this is granted per terminal individually rather than
+ * assumed from any other capability.
+ */
+static const struct tty_feature tty_feature_imagescroll = {
+	"imagescroll",
+	tty_feature_image_capabilities,
+	TERM_IMAGESCROLL
+};
+#endif
+
 /* Terminal supports the OSC 9;4 progress bar. */
 static const char *const tty_feature_progressbar_capabilities[] = {
 	"Spb=\\E]9;4;%p1%d;%p2%d\\E\\\\",
@@ -399,6 +445,12 @@ static const struct tty_feature *const tty_features[] = {
 	&tty_feature_extkeys,
 	&tty_feature_focus,
 	&tty_feature_ignorefkeys,
+	&tty_feature_kitty,
+#ifdef ENABLE_IMAGES
+	&tty_feature_image_quadrants,
+	&tty_feature_image_sextants,
+	&tty_feature_imagescroll,
+#endif
 	&tty_feature_margins,
 	&tty_feature_mouse,
 	&tty_feature_osc7,
@@ -637,7 +689,9 @@ tty_default_features(struct client *c, const char *name, u_int version)
 			      "extkeys,"
 			      "focus,"
 		  	      "hyperlinks,"
+			      "imagescroll,"
 			      "margins,"
+			      "sixel,"
 			      "usstyle"
 		},
 		{ .name = "ghostty",
@@ -646,13 +700,28 @@ tty_default_features(struct client *c, const char *name, u_int version)
 			      "cstyle,"
 			      "extkeys,"
 			      "focus,"
+			      "imagescroll,"
 			      "margins,"
 			      "overline,"
 			      "hyperlinks,"
 			      "osc7,"
 			      "sync,"
 			      "usstyle,"
-			      "progressbar"
+			      "progressbar,"
+			      "kitty"
+		},
+		{ .name = "kitty",
+		  .features = TTY_FEATURES_BASE_MODERN_XTERM ","
+			      "ccolour,"
+			      "cstyle,"
+			      "extkeys,"
+			      "focus,"
+			      "hyperlinks,"
+			      "imagescroll,"
+			      "osc7,"
+			      "sync,"
+			      "usstyle,"
+			      "kitty"
 		},
 		{ .name = "Rio",
 		  .features = TTY_FEATURES_BASE_MODERN_XTERM ","