Enable WebRTC screen capturer in WebEngine

This is disabled by default, and without it screen sharing doesn't work on Wayland
Port of https://invent.kde.org/network/konqueror/-/merge_requests/113

BUG: 450839
(cherry picked from commit 02feca74ce)

src/lib/app/mainapplication.cpp

@@ -151,6 +151,10 @@ MainApplication::MainApplication(int &argc, char** argv)
     }
 #endif
 
+    QByteArray flags = qgetenv("QTWEBENGINE_CHROMIUM_FLAGS");
+    flags.append(" --enable-features=WebRTCPipeWireCapturer");
+    qputenv("QTWEBENGINE_CHROMIUM_FLAGS", flags);
+
     QUrl startUrl;
     QString startProfile;
     QStringList messages;