[GTK][WPE] Do not use real time priority for the display link thread

https://bugs.webkit.org/show_bug.cgi?id=268900

Reviewed by Michael Catanzaro.

In some cases it can cause the rtkit daemon to kill the ui process due
to CPU usage limit.

* Source/WebKit/UIProcess/glib/DisplayVBlankMonitor.cpp:
(WebKit::DisplayVBlankMonitor::startThreadIfNeeded):

Canonical link: https://commits.webkit.org/274210@main

Source/WebKit/UIProcess/glib/DisplayVBlankMonitor.cpp

@@ -97,7 +97,7 @@ bool DisplayVBlankMonitor::startThreadIfNeeded()
             if (active)
                 m_handler();
         }
-    }, ThreadType::Graphics, Thread::QOS::UserInteractive);
+    }, ThreadType::Graphics, Thread::QOS::Default);
     return true;
 }