Add imagescroll feature and capability.

tmux.1

@@ -5194,6 +5194,8 @@ Supports extended keys.
 Supports focus reporting.
 .It hyperlinks
 Supports OSC 8 hyperlinks.
+.It imagescroll
+Moves image content with scroll regions.
 .It ignorefkeys
 Ignore function keys from
 .Xr terminfo 5
@@ -8984,6 +8986,8 @@ These are set automatically if the
 capability is present.
 .It Em \&Hls
 Set or clear a hyperlink annotation.
+.It Em \&Iscrl
+Indicates that the terminal moves image content with scroll regions.
 .It Em \&Nobr
 Tell
 .Nm