patch 9.0.1684: Update libvterm to rev 839

Problem: libvterm slightly outdated
Solution: Update libvterm from rev 818 to rev 839

Notable fix: libvterm now handles DECSM/DECRM with multiple arguents,
so several ncurses programs (e.g. nnn) can enable mouse properly when
run in Vim's terminal in XTerm.

closes: #12746

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
diff --git a/src/libvterm/t/69screen_reflow.test b/src/libvterm/t/69screen_reflow.test
index 278cc5b..eb7e4e4 100644
--- a/src/libvterm/t/69screen_reflow.test
+++ b/src/libvterm/t/69screen_reflow.test
@@ -77,3 +77,12 @@
   ?lineinfo 3 =
   ?screen_row 3 = "> "
   ?cursor = 3,2
+
+!Cursor goes missing
+# For more context: https://github.com/neovim/neovim/pull/21124
+RESET
+RESIZE 5,5
+RESIZE 3,1
+PUSH "\x1b[2;1Habc\r\n\x1b[H"
+RESIZE 1,1
+  ?cursor = 0,0