Add wxBitmap(wxImage, wxDC) ctor to all ports

This ctor was previously present only in wxMSW, make it available in all
ports to allow the same code to compile everywhere.

In most of them wxDC argument is simply ignored, but in wxGTK and wxOSX
it is used to assign the appropriate scale factor for the new bitmap.

Enable previously wxMSW-only unit test checking for this.

include/wx/osx/bitmap.h

@@ -114,6 +114,7 @@ public:
 
     // Convert from wxImage:
     wxBitmap(const wxImage& image, int depth = -1, double scale = 1.0);
+    wxBitmap(const wxImage& image, const wxDC& dc);
 
     // Convert from wxIcon
     wxBitmap(const wxIcon& icon) { CopyFromIcon(icon); }
@@ -236,6 +237,9 @@ protected:
     virtual wxGDIRefData *CloneGDIRefData(const wxGDIRefData *data) const wxOVERRIDE;
 
     virtual bool DoCreate(const wxSize& sz, double scale, int depth) wxOVERRIDE;
+
+private:
+    void InitFromImage(const wxImage& image, int depth, double scale);
 };
 
 #endif // _WX_BITMAP_H_