Improve wxD2DContext::GetDPI and wxGDIPlusContext::GetDPI

If there is a valid wxWindow, use its DPI. Otherwise use a dedicated function
of the context to get the DPI. Don't use the common wxGraphicsContext::GetDPI
because this will return hard-coded 72 when there is no valid wxWindow.

src/msw/graphics.cpp

@@ -458,6 +458,7 @@ public:
     virtual void GetPartialTextExtents(const wxString& text, wxArrayDouble& widths) const wxOVERRIDE;
     virtual bool ShouldOffset() const wxOVERRIDE;
     virtual void GetSize( wxDouble* width, wxDouble *height );
+    virtual void GetDPI(wxDouble* dpiX, wxDouble* dpiY) const wxOVERRIDE;
 
     Graphics* GetGraphics() const { return m_context; }
 
@@ -2397,6 +2398,26 @@ void wxGDIPlusContext::GetSize( wxDouble* width, wxDouble *height )
     *height = m_height;
 }
 
+void wxGDIPlusContext::GetDPI(wxDouble* dpiX, wxDouble* dpiY) const
+{
+    if ( GetWindow() )
+    {
+        const wxSize dpi = GetWindow()->GetDPI();
+
+        if ( dpiX )
+            *dpiX = dpi.x;
+        if ( dpiY )
+            *dpiY = dpi.y;
+    }
+    else
+    {
+        if ( dpiX )
+            *dpiX = GetGraphics()->GetDpiX();
+        if ( dpiY )
+            *dpiY = GetGraphics()->GetDpiY();
+    }
+}
+
 //-----------------------------------------------------------------------------
 // wxGDIPlusPrintingContext implementation
 //-----------------------------------------------------------------------------

src/msw/graphicsd2d.cpp

@@ -4575,11 +4575,26 @@ void wxD2DContext::Flush()
 }
 
 void wxD2DContext::GetDPI(wxDouble* dpiX, wxDouble* dpiY) const
+{
+    if ( GetWindow() )
+    {
+        const wxSize dpi = GetWindow()->GetDPI();
+
+        if ( dpiX )
+            *dpiX = dpi.x;
+        if ( dpiY )
+            *dpiY = dpi.y;
+    }
+    else
     {
         FLOAT x, y;
         GetRenderTarget()->GetDpi(&x, &y);
-    if (dpiX != NULL) *dpiX = x;
-    if (dpiY != NULL) *dpiY = y;
+
+        if ( dpiX )
+            *dpiX = x;
+        if ( dpiY )
+            *dpiY = y;
+    }
 }
 
 //-----------------------------------------------------------------------------