Expose the ThreadedOpenGL cap to apps

Applications have no public API to check for the ThreadedOpenGL flag
reported by the platform plugins. Add a static function to QOpenGLContext
to give interested applications a way to decide if they should do
rendering on separate threads.

Task-number: QTBUG-44208
Change-Id: I629332512ab31fe49f842065a911270cd6cea7aa
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>

src/gui/kernel/qopenglcontext.cpp

@@ -1184,6 +1184,21 @@ bool QOpenGLContext::isOpenGLES() const
     return format().renderableType() == QSurfaceFormat::OpenGLES;
 }
 
+/*!
+  Returns \c true if the platform supports OpenGL rendering outside the main (gui)
+  thread.
+
+  The value is controlled by the platform plugin in use and may also depend on the
+  graphics drivers.
+
+  \since 5.5
+ */
+bool QOpenGLContext::supportsThreadedOpenGL()
+{
+    Q_ASSERT(qGuiApp);
+    return QGuiApplicationPrivate::instance()->platformIntegration()->hasCapability(QPlatformIntegration::ThreadedOpenGL);
+}
+
 /*!
     \internal
 */

src/gui/kernel/qopenglcontext.h

@@ -198,6 +198,8 @@ public:
 
     bool isOpenGLES() const;
 
+    static bool supportsThreadedOpenGL();
+
 Q_SIGNALS:
     void aboutToBeDestroyed();