vcl opengl: avoid task priority warning on cursor blink

warn:vcl.schedule:22358:22358:include/vcl/task.hxx:107: Priority will just change after next schedule!

Just don't set the priority if it's already set to the correct value, so
we can avoid a warning each time the visible cursor is shown or hidden.

Change-Id: Ie11164db0af4cfba06a620c9b2426cb903af3887
Reviewed-on: https://gerrit.libreoffice.org/68907
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins

vcl/opengl/gdiimpl.cxx

@@ -63,7 +63,8 @@ public:
     virtual void Invoke() override
     {
         m_pImpl->doFlush();
-        SetPriority( TaskPriority::HIGHEST );
+        if (GetPriority() != TaskPriority::HIGHEST)
+            SetPriority(TaskPriority::HIGHEST);
         Stop();
     }
 };