Handle wxDC scale factor in wxBitmap ctor taking wxDC in wxMSW

When using wxBitmap ctor taking wxDC, the bitmap should inherit the
scale factor of the DC, both because it's already the case in the other
ports and because it makes sense to do it.

Add a unit test checking that this is the case in all ports now.

src/msw/bitmap.cpp

@@ -755,7 +755,14 @@ bool wxBitmap::Create(int width, int height, const wxDC& dc)
 {
     wxCHECK_MSG( dc.IsOk(), false, wxT("invalid HDC in wxBitmap::Create()") );
 
-    return DoCreate(width, height, -1, dc.GetHDC());
+    const double scale = dc.GetContentScaleFactor();
+
+    if ( !DoCreate(wxRound(width*scale), wxRound(height*scale), -1, dc.GetHDC()) )
+        return false;
+
+    GetBitmapData()->m_scaleFactor = scale;
+
+    return true;
 }
 
 bool wxBitmap::CreateWithDIPSize(const wxSize& size, double scale, int depth)