Remove usage of QOpenGLFunctions and use GLEW instead.

Qt's OpenGL interface was causing problems on Windows since it doesn't mix
well with ANGLE.

Relates to issue #50.

examples/Basic/OpenGLWidget.h

@@ -26,12 +26,10 @@ distribution.
 
 #include "PolyVoxCore/SurfaceMesh.h"
 
-#include <QOpenGLFunctions_3_1>
+#include "glew/glew.h"
 
 #include <QGLWidget>
 #include <QOpenGLShaderProgram>
-#include <QOpenGLVertexArrayObject>
-#include <QOpenGLBuffer>
 
 class OpenGLWidget : public QGLWidget
 {
@@ -60,7 +58,6 @@ private:
 	GLuint vertexArrayObject;
 	
 	QOpenGLShaderProgram shader;
-	QOpenGLFunctions_3_1* gl;
 
 	//Mouse data
 	QPoint m_LastFrameMousePos;