[client] input: stop absolute cursor feedback

X11's legacy guest-position synchronization warps the host cursor to
delayed guest updates. With absolute input this feeds old positions back
through cooked motion and pulls the cursor away from window edges.

Skip display-server guest synchronization when the active input is
absolute. Relative input and explicit uncapture alignment are unchanged.

client/tests/CMakeLists.txt

@@ -120,6 +120,7 @@ set(MOUSE_CASES
   surface-exit
   exit-immediate
   exit-guest
+  absolute-sync
   exit-reentry
   view-immediate
   capture-pending