qCritical if the user creates a raster surface

Raster surfaces are not allowed due to performance problems. A
qCritical message is shown if the user tries to use the plugin in a
widget application.

[ChangeLog][QtWebGL] The application shows an error message when
trying to create a QBackingStore in an application.

Task-number: QTBUG-65054
Change-Id: I78bfd2b2714d7dce1f802b1864216655e2d74406
Reviewed-by: Edward Welbourne <[email protected]>

Author: jsfdez

src/plugins/platforms/webgl/qwebglintegration.cpp

@@ -179,6 +179,7 @@ QPlatformTheme *QWebGLIntegration::createPlatformTheme(const QString &name) cons
 QPlatformBackingStore *QWebGLIntegration::createPlatformBackingStore(QWindow *window) const
 {
     Q_UNUSED(window);
+    qCCritical(lcWebGL, "WebGL QPA platform plugin: Raster surfaces are not supported");
     return nullptr;
 }