qt: gl-core; Use 4.1 core macOS & get glsl version from context

MacOS will use OpenGL 2.1 unless requested 4.1 core. Shader language
version is now set to what is reported by the driver.

src/qt/qt_openglrenderer.hpp

@@ -77,6 +77,8 @@ private:
     OpenGLOptions *options;
     QTimer        *renderTimer;
 
+    QString glslVersion;
+
     bool isInitialized = false;
     bool isFinalized   = false;