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Consolidate the image work through 250f96b4b onto the redraw-damage-rectangles-new base, excluding the superseded redraw implementation and its historical merges.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-scroll-no-erase.sh

@@ -0,0 +1,107 @@
+#!/bin/sh
+
+# Regression test for a tmux bug (not a terminal bug): image_redraw_scroll()
+# (image.c) marks the whole scroll region as damage on every scroll. For a
+# client whose terminal has imagescroll, the terminal has already moved the
+# image (and the text) along with the scroll, but the damage was still
+# composed: the image was not retransmitted, but the ordinary text pass
+# drew blank grid content directly over the image's cells and erased a
+# correctly scrolled image. This is what a user saw as "the image
+# disappears when the pane scrolls in Windows Terminal" even with
+# imagescroll and margins both granted, and could never be reproduced by
+# any terminal-side test because the bug is entirely server side.
+#
+# redraw_client_damage_rect() (screen-redraw.c) now composes nothing for a
+# scroll the client's terminal is trusted to have done itself, logging
+# "composing damage" only when it draws. Nothing being drawn is what keeps
+# the image, so the absence of that log line for the scroll is checked.
+#
+# The image is placed comfortably mid-pane (not at the very top) and wide
+# enough to span the whole pane, so it is still on screen - not already
+# scrolled into history - by the time a real scroll's damage gets composed.
+
+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$$ -vv -f/dev/null"
+TMUX2="$TEST_TMUX -LtestB$$ -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
+
+DIR=$(mktemp -d) || exit 1
+cd "$DIR" || exit 1
+trap "cleanup; cd /; rm -rf $DIR" 0 1 15
+
+WIDTH=40
+HEIGHT=20
+ROW=10
+
+# A sixel image exactly WIDTH cells wide (at a typical ~9px cell, 360px) and
+# one cell tall, placed at screen row ROW - comfortably mid-pane, not at
+# the very top, so it stays visible (not yet scrolled into history) through
+# the first several scrolls.
+HEADER='\033Pq"1;1;360;18#0;2;100;100;100#0!360~-!360~-!360B\033\\'
+
+# Pad with blank lines after the image so the shell prompt lands at the
+# bottom of the pane - without this, the very first scroll from a
+# not-yet-settled cursor position falls back to tty_redraw_region() (a
+# different, unrelated path that the trusted-scroll skip never applies to
+# anyway), and the test would not exercise what it is meant to.
+PAD=$((HEIGHT - ROW - 2))
+$TMUX new-session -d -s inner -x $WIDTH -y $HEIGHT \
+	"printf '\\033[$((ROW + 1));1H$HEADER'; i=0; while [ \$i -lt $PAD ]; do echo; i=\$((i + 1)); done; exec sh" ||
+	exit 1
+sleep 0.3
+
+[ "$($TMUX display-message -p '#{image_support}')" = 0 ] && exit 0
+$TMUX set -g pane-scrollbars on || exit 1
+$TMUX set -as terminal-features ',*:sixel' || exit 1
+$TMUX set -as terminal-features ',*:margins' || exit 1
+$TMUX set -as terminal-features ',*:imagescroll' || exit 1
+
+$TMUX2 new-session -d -x $WIDTH -y $HEIGHT || exit 1
+OUTER=$($TMUX2 list-panes -F '#{pane_id}' | head -1)
+[ -n "$OUTER" ] || fail "No outer pane."
+$TMUX2 set -as terminal-features ',*:sixel@' || fail "disable outer sixel failed"
+TMP=$(mktemp)
+$TMUX2 pipe-pane -t "$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 1
+
+grep -qa '"1;1;360;18' "$TMP" || fail "sanity: image never reached the client"
+
+LOG=$(ls tmux-server*.log 2>/dev/null | head -1)
+[ -n "$LOG" ] || fail "sanity: no server -vv log was produced"
+BEFORE=$(wc -l <"$LOG")
+: >"$TMP"
+
+# The image (row ROW, 10 of 20) has plenty of room above it - this stays
+# well within the visible pane for the whole burst.
+$TMUX send-keys -t inner -l "yes | head -n 10" || fail "send scroll failed"
+$TMUX send-keys -t inner Enter || fail "send enter failed"
+sleep 1
+
+n_dcs=$(grep -ac "$(printf '\033P')" "$TMP")
+[ "$n_dcs" -eq 0 ] ||
+	fail "sanity: image was retransmitted ($n_dcs times) - imagescroll's trust did not engage, so this run cannot test what it is meant to"
+
+tail -n +$((BEFORE + 1)) "$LOG" | grep -q "redraw_client_damage_rect: .* composing damage" &&
+	fail "damage was composed for a scroll the terminal is trusted to have done - this would draw over the image and erase it"
+
+exit 0