windows opengl: Set PFD_DOUBLEBUFFER even for single buffer rendering.

Otherwise I'm getting lots of errers when dealing with virtual devices...

Change-Id: I34e145817d21dfc2d30fa9ffdfedfa714467d105

vcl/opengl/gdiimpl.cxx

@@ -1478,6 +1478,7 @@ bool OpenGLSalGraphicsImpl::drawGradient(const tools::PolyPolygon& rPolyPoly,
         glDisable( GL_STENCIL_TEST );
     PostDraw();
 
+    CHECK_GL_ERROR();
     return true;
 }
 
@@ -1496,6 +1497,8 @@ void OpenGLSalGraphicsImpl::endPaint()
         maContext.makeCurrent();
         glFlush();
     }
+
+    CHECK_GL_ERROR();
 }
 
 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */

vcl/source/opengl/OpenGLContext.cxx

@@ -767,8 +767,9 @@ bool OpenGLContext::ImplInit()
         0, 0, 0                         // Layer Masks Ignored
     };
 
-    if (mbUseDoubleBufferedRendering)
-        PixelFormatFront.dwFlags |= PFD_DOUBLEBUFFER;
+    // interestingly we need this flag being set even if we use single buffer
+    // rendering - otherwise we get errors with virtual devices
+    PixelFormatFront.dwFlags |= PFD_DOUBLEBUFFER;
 
     if (mbRequestVirtualDevice)
         PixelFormatFront.dwFlags |= PFD_DRAW_TO_BITMAP;