make it possible to select the OpenGL rendering on windows

Change-Id: Id60b8121a890e6d1f46bd671549ac681f42b7660

include/vcl/opengl/OpenGLContext.hxx

@@ -22,9 +22,12 @@
 #  include "GL/glxew.h"
 #  include <postx.h>
 #elif defined( _WIN32 )
+#ifndef INCLUDED_PRE_POST_WIN_H
+#define INCLUDED_PRE_POST_WIN_H
 #  include "prewin.h"
 #  include "postwin.h"
 #endif
+#endif
 
 #if defined( _WIN32 )
 #include <GL/glext.h>

vcl/inc/win/svsys.h

@@ -21,9 +21,12 @@
 #define INCLUDED_VCL_INC_WIN_SVSYS_H
 
 #ifdef WNT
+#ifndef INCLUDED_PRE_POST_WIN_H
+#define INCLUDED_PRE_POST_WIN_H
 #include <prewin.h>
 #include <postwin.h>
 #endif
+#endif
 
 #endif // INCLUDED_VCL_INC_WIN_SVSYS_H
 

vcl/win/source/gdi/salgdi.cxx

@@ -34,6 +34,7 @@
 
 #include "salgdiimpl.hxx"
 #include "gdiimpl.hxx"
+#include "openglgdiimpl.hxx"
 
 #define DITHER_PAL_DELTA                51
 #define DITHER_PAL_STEPS                6
@@ -581,6 +582,11 @@ WinSalGraphics::WinSalGraphics():
     mnFontKernPairCount = 0;
     mbFontKernInit      = FALSE;
     mnPenWidth          = GSL_PEN_WIDTH;
+    static const char* pEnv = getenv("USE_OPENGL");
+    if (pEnv)
+    {
+        mpImpl.reset(new OpenGLSalGraphicsImpl());
+    }
 }
 
 WinSalGraphics::~WinSalGraphics()