Merge image_merge into 4902-image-support

Keep retained indexed SIXEL data and the bounded per-terminal output cache.
Use the cleaned-up decoder and RGBA encoder from image_merge, including
parser memory limits, pixel-size handling and references for pending output.
Match retained SIXEL scaling to the shared image cell footprint and emit
black definitions for undefined colour registers.

Keep Kitty input, output, layers and placeholders. Adapt backend selection,
flags, drawing and cleanup to the callbacks and Iscrl capability. Keep the
pane-status repaint with the requested XXX comment.

Replace the elaborate fallback with image_merge's implementation, removing
its caches, sample structures, quadrant/sextant features and ACS additions.
Keep the image_merge regressions and adapt the Kitty tests to the simpler
fallback. Use skip status 101 and disable Python bytecode for image tests.

Small changes still outside image_merge:
- Guard zero-height synchronized redraws before bit_nset.
- Select the image backend when the tty opens.
- Enable imagescroll by default for Kitty and Ghostty.
- Keep the explanatory manual-window pixel-size comment (comment only).
- Keep the additional SIXEL fixtures and visual test tools.

Validation: image-enabled and disabled builds; 31 image and 13 related
regressions pass normally and under ASAN. All image tests skip without
images. Focused palette, parser, scaling, transparency and Kitty layer
tests also pass after the retained-output adjustments. Diff checks clean.

regress/image-scroll-no-text-redraw.sh

@@ -1,87 +1,17 @@
 #!/bin/sh
 
-# When the client's terminal has the imagescroll feature, tmux trusts it to
-# have moved everything in the scrolled region - text and image alike - so
-# a scroll must not cause any existing text to be redrawn. It used to skip
-# only the image phases of the damage redraw and still redraw all of the
-# text around them.
-#
-# Without imagescroll the images must be redrawn, and the text around them
-# is redrawn too, so the same scroll must resend it.
-#
-# The feature only takes effect for a client attached after it is set, so
-# each case starts a new server and client.
+# Adapt the PR's Kitty marker check to SIXEL. Native image scrolling must
+# skip the surrounding text repaint as well as image retransmission.
 
 PATH=/bin:/usr/bin
 TERM=screen
-LC_ALL=C.UTF-8
-export TERM LC_ALL
-
+export TERM
 [ -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
-}
+python3 -B - "$TEST_TMUX" <<'PY'
+import sys
+from image_scroll import check_scroll
 
-TMP=$(mktemp)
-trap "cleanup; rm -f $TMP" 0 1 15
-
-HEADER=$(printf '\033_Ga=T,q=2,f=32,s=1,v=1,i=424243;/wAA/w==\033\\')
-
-# Scroll a pane containing an image and marker text on a client with the
-# given terminal features, and set N to the number of marker lines sent to
-# the terminal.
-scroll_markers()
-{
-	cleanup
-
-	# Marker text around an image, filling the pane so that the next
-	# newlines scroll it while the image stays in view.
-	$TMUX new-session -d -s inner -x 40 -y 20 \
-	    "i=0; while [ \$i -lt 5 ]; do printf 'MARKER%02d\n' \$i; i=\$((i + 1)); done; printf '$HEADER'; while [ \$i -lt 17 ]; do printf 'MARKER%02d\n' \$i; i=\$((i + 1)); done; exec sh" ||
-		exit 1
-	sleep 0.3
-	[ "$($TMUX display-message -p '#{image_support}')" = 0 ] && exit 0
-	$TMUX set -as terminal-features "$1" || 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."
-	$TMUX2 set -as terminal-features ',*:kittygraphics@' ||
-	    fail "disable outer kitty failed"
-	: >"$TMP"
-	$TMUX2 pipe-pane -t "$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"
-
-	: >"$TMP"
-	$TMUX send-keys -t inner -l "printf '\n\n\n'" ||
-	    fail "send scroll failed"
-	$TMUX send-keys -t inner Enter || fail "send enter failed"
-	sleep 1
-	N=$(grep -ac MARKER "$TMP")
-}
-
-scroll_markers ',*:kittygraphics,*:imagescroll'
-[ "$N" -eq 0 ] ||
-	fail "existing text was redrawn ($N lines) scrolling with imagescroll"
-
-scroll_markers ',*:kittygraphics'
-[ "$N" -gt 0 ] ||
-	fail "text was not redrawn scrolling without imagescroll"
-
-exit 0
+check_scroll(sys.argv[1], True)
+check_scroll(sys.argv[1], False)
+PY