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Consolidate the image work through 250f96b4b onto the redraw-damage-rectangles-new base, excluding the superseded redraw implementation and its historical merges.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

regress/image-kitty-region-scroll.sh

@@ -0,0 +1,101 @@
+#!/bin/sh
+
+# Regression test for the imagescroll terminal-feature (tty-features.c),
+# Kitty side - see image-sixel-region-scroll.sh for the SIXEL side of the
+# same feature.
+#
+# The Kitty backend used to hardcode IMAGE_BACKEND_SCROLLS unconditionally
+# (image.c), so tmux always trusted the terminal to have moved a Kitty
+# placement along with the rest of a scrolling region, with no way to turn
+# that assumption off. At least Windows Terminal does not actually do
+# this: it drops the placement entirely on a plain newline- or
+# reverse-index-driven scroll (though not on a delete-line/insert-line
+# scroll simulation, which goes through a different, always-redraw path).
+# imagescroll now gates Kitty exactly like SIXEL: scrolling must not
+# retransmit the placement when the client's terminal has imagescroll (the
+# previously unconditional behaviour), and must retransmit it without (the
+# default for any terminal not individually confirmed and added to
+# tty-features.c's table) - the escape hatch this test exists to prove is
+# actually wired up for Kitty too.
+
+PATH=/bin:/usr/bin
+TERM=screen
+LC_ALL=C.UTF-8
+export TERM LC_ALL
+
+[ -z "$TEST_TMUX" ] && TEST_TMUX=$(readlink -f ../tmux)
+TMUX="$TEST_TMUX -LtestA$$ -f/dev/null"
+TMUX2="$TEST_TMUX -LtestB$$ -f/dev/null"
+
+cleanup()
+{
+	$TMUX kill-server >/dev/null 2>&1
+	$TMUX2 kill-server >/dev/null 2>&1
+}
+fail()
+{
+	echo "$*" >&2
+	cleanup
+	exit 1
+}
+
+cleanup
+
+TMP=$(mktemp)
+trap "cleanup; rm -f $TMP" 0 1 15
+
+IMAGEID=424242
+HEADER=$(printf '\033_Ga=T,q=2,f=32,s=1,v=1,i=%s;/wAA/w==\033\\' "$IMAGEID")
+
+# A small image near the top of a tall pane, then enough plain output below
+# it to force several ordinary (no DECSTBM sub-region) linefeed scrolls,
+# each one shifting the image rows along with everything else.
+$TMUX new-session -d -s inner -x 40 -y 20 "printf '$HEADER'; exec sh" ||
+	exit 1
+sleep 0.3
+
+[ "$($TMUX display-message -p '#{image_support}')" = 0 ] && exit 0
+$TMUX set -as terminal-features ',*:kitty' || exit 1
+
+$TMUX2 new-session -d -x 40 -y 20 || exit 1
+OUTER=$($TMUX2 list-panes -F '#{pane_id}' | head -1)
+[ -n "$OUTER" ] || fail "No outer pane."
+$TMUX2 set -as terminal-features ',*:kitty@' || fail "disable outer kitty failed"
+$TMUX2 pipe-pane -t "$OUTER" -O "cat >$TMP" || fail "pipe-pane failed"
+$TMUX2 send-keys -t "$OUTER" -l "$TMUX attach -t inner" || fail "send attach failed"
+$TMUX2 send-keys -t "$OUTER" Enter || fail "send enter failed"
+sleep 2
+
+grep -qa "$(printf '\033_Ga=p')" "$TMP" || fail "sanity: image never reached the client"
+
+# --- Phase 1: imagescroll granted - expect no APC. ---
+$TMUX set -as terminal-features ',*:imagescroll' || fail "grant imagescroll failed"
+sleep 0.5
+: >"$TMP"
+$TMUX send-keys -t inner -l "yes | head -n 30" || fail "send scroll failed"
+$TMUX send-keys -t inner Enter || fail "send enter failed"
+sleep 1
+
+n_on=$(grep -ac "$(printf '\033_Ga=p')" "$TMP")
+[ "$n_on" -eq 0 ] ||
+	fail "image was retransmitted ($n_on times) scrolling with imagescroll granted"
+
+# --- Phase 2: imagescroll not granted (the default) - expect the image
+# back, then more scrolling to redraw it. ---
+$TMUX send-keys -t inner -l "printf '$HEADER'" || fail "resend image failed"
+$TMUX send-keys -t inner Enter || fail "send enter failed"
+sleep 1.5
+grep -qa "$(printf '\033_Ga=p')" "$TMP" || fail "sanity: image did not reappear before phase 2"
+
+$TMUX set -as terminal-features ',*:imagescroll@' || fail "revoke imagescroll failed"
+sleep 0.5
+: >"$TMP"
+$TMUX send-keys -t inner -l "yes | head -n 30" || fail "send scroll failed"
+$TMUX send-keys -t inner Enter || fail "send enter failed"
+sleep 1
+
+n_off=$(grep -ac "$(printf '\033_Ga=p')" "$TMP")
+[ "$n_off" -gt 0 ] ||
+	fail "image was not retransmitted scrolling without imagescroll - expected the default always-redraw behaviour"
+
+exit 0