[client] X11: release pointer after capture

Normal input now uses absolute positioning, so retaining the XI pointer
grab after capture leaves no later path to release it.

Always release the grab so leaving the window works again and the
uncapture path can align the host cursor to the guest position.

client/tests/CMakeLists.txt

@@ -126,6 +126,7 @@ set(MOUSE_CASES
   capture-revoked
   capture-release
   capture-align
+  x11-capture-exit
   capture-fallback
   rotate-scale
   geometry