inherit unversioned QOpenGLFunctions class for GLWaveformRenderer

The unversioned QOpenGLFunctions class provides the baseline
OpenGL functions available in Qt, namely OpenGL ES 2.0. Only
the legacy OpenGL waveform renderers require desktop OpenGL 2.1;
the rest of the OpenGL code works fine with OpenGL ES 2.0.

src/waveform/renderers/glwaveformrenderer.h

@@ -2,7 +2,7 @@
 
 #include <QtGui/qtgui-config.h> // for QT_NO_OPENGL and QT_OPENGL_ES_2
 
-#if !defined(QT_NO_OPENGL) && !defined(QT_OPENGL_ES_2)
+#if !defined(QT_NO_OPENGL)
 
 #include <QOpenGLFunctions>
 
@@ -13,4 +13,4 @@ class GLWaveformRenderer : protected QOpenGLFunctions {
     }
 };
 
-#endif // !defined(QT_NO_OPENGL) && !defined(QT_OPENGL_ES_2)
+#endif // !defined(QT_NO_OPENGL)