Add support for the kitty graphics protocol

Kitty-capable terminals (kitty, ghostty, and others) can display
inline images via APC escape sequences.

Kitty's image support uses a passthrough model: APC sequences from
programs running inside panes are relayed verbatim to the outer
terminal with cursor positioning adjusted for the pane offset.

The outer terminal handles all image rendering and therefore itself must
be kitty-aware.

tty-features.c

@@ -357,6 +357,17 @@ static const struct tty_feature tty_feature_sixel = {
 	TERM_SIXEL
 };
 
+/* Terminal has kitty graphics protocol capability. */
+static const char *const tty_feature_kitty_capabilities[] = {
+	"Kty",
+	NULL
+};
+static const struct tty_feature tty_feature_kitty = {
+	"kitty",
+	tty_feature_kitty_capabilities,
+	TERM_KITTY
+};
+
 /* Available terminal features. */
 static const struct tty_feature *const tty_features[] = {
 	&tty_feature_256,
@@ -368,6 +379,7 @@ static const struct tty_feature *const tty_features[] = {
 	&tty_feature_extkeys,
 	&tty_feature_focus,
 	&tty_feature_ignorefkeys,
+	&tty_feature_kitty,
 	&tty_feature_margins,
 	&tty_feature_mouse,
 	&tty_feature_osc7,
@@ -500,7 +512,19 @@ tty_default_features(int *feat, const char *name, u_int version)
 		   */
 		  .features = TTY_FEATURES_BASE_MODERN_XTERM
 			      ",ccolour,cstyle,extkeys,focus"
-		}
+		},
+#ifdef ENABLE_KITTY
+		{ .name = "kitty",
+		  .features = TTY_FEATURES_BASE_MODERN_XTERM
+			      ",ccolour,cstyle,extkeys,focus,sync,hyperlinks"
+			      ",kitty"
+		},
+		{ .name = "ghostty",
+		  .features = TTY_FEATURES_BASE_MODERN_XTERM
+			      ",ccolour,cstyle,extkeys,focus,sync,hyperlinks"
+			      ",kitty"
+		},
+#endif
 	};
 	u_int	i;