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Consolidate the image work through 250f96b4b onto the redraw-damage-rectangles-new base, excluding the superseded redraw implementation and its historical merges.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

configure.ac

@@ -541,15 +541,35 @@ if test "x$enable_cgroups" = xyes; then
 	fi
 fi
 
-# Enable sixel support.
+# Enable image support.
+AC_ARG_ENABLE(
+	images,
+	AS_HELP_STRING(--enable-images, enable Kitty/SIXEL/ASCII images)
+)
+
+# Keep the old option as an alias for one compatibility cycle.
 AC_ARG_ENABLE(
 	sixel,
-	AS_HELP_STRING(--enable-sixel, enable sixel images)
+	AS_HELP_STRING(--enable-sixel, deprecated alias for --enable-images)
 )
 if test "x$enable_sixel" = xyes; then
-	AC_DEFINE(ENABLE_SIXEL)
+	AC_MSG_WARN([--enable-sixel is deprecated, use --enable-images instead to enable.])
+	enable_images=yes
 fi
-AM_CONDITIONAL(ENABLE_SIXEL, [test "x$enable_sixel" = xyes])
+if test "x$enable_images" = xyes; then
+	PKG_CHECK_MODULES(
+		IMAGE,
+		[libpng >= 1.6 zlib],
+		[
+			AM_CPPFLAGS="$IMAGE_CFLAGS $AM_CPPFLAGS"
+			CPPFLAGS="$AM_CPPFLAGS $SAVED_CPPFLAGS"
+			LIBS="$IMAGE_LIBS $LIBS"
+		],
+		AC_MSG_ERROR("image support requires libpng 1.6 and zlib")
+	)
+	AC_DEFINE(ENABLE_IMAGES)
+fi
+AM_CONDITIONAL(ENABLE_IMAGES, [test "x$enable_images" = xyes])
 
 # Check for b64_ntop. If we have b64_ntop, we assume b64_pton as well.
 AC_MSG_CHECKING(for b64_ntop)
@@ -1148,10 +1168,10 @@ if test "x$enable_asan" = xyes; then
 else
 	AC_MSG_NOTICE([ASAN: off])
 fi
-if test "x$enable_sixel" = xyes; then
-	AC_MSG_NOTICE([SIXEL: on])
+if test "x$enable_images" = xyes; then
+	AC_MSG_NOTICE([images: on])
 else
-	AC_MSG_NOTICE([SIXEL: off])
+	AC_MSG_NOTICE([images: off])
 fi
 if test "x$enable_debug" = xyes; then
 	AC_MSG_NOTICE([debug: on])