Fix OpenGL canvas freeze under Windows.

common/gal/opengl/opengl_gal.cpp

@@ -124,7 +124,7 @@ void OPENGL_GAL::BeginDrawing()
 {
     SetCurrent( *glContext );
 
-    clientDC = new wxPaintDC( this );
+    clientDC = new wxClientDC( this );
 
     // Initialize GLEW, FBOs & VBOs
     if( !isGlewInitialized )

include/gal/opengl/opengl_gal.h

@@ -252,7 +252,7 @@ private:
     static const int    CIRCLE_POINTS   = 64;   ///< The number of points for circle approximation
     static const int    CURVE_POINTS    = 32;   ///< The number of points for curve approximation
 
-    wxPaintDC*              clientDC;               ///< Drawing context
+    wxClientDC*             clientDC;               ///< Drawing context
     static wxGLContext*     glContext;              ///< OpenGL context of wxWidgets
     wxWindow*               parentWindow;           ///< Parent window
     wxEvtHandler*           mouseListener;