Add wxWindow::GetDPIScaleFactor()

This function replaces some uses of GetContentScaleFactor(), where a
factor greater than 1 must be used even under the platforms not doing
any logical/physical pixel mapping, such as MSW.

For now GetContentScaleFactor() is still unchanged, but it will return 1
for such platforms in the future and adding GetDPIScaleFactor() allows
to avoid changing the behaviour of the code which relied on its current
behaviour.

src/html/htmlwin.cpp

@@ -493,7 +493,7 @@ bool wxHtmlWindow::DoSetPage(const wxString& source)
 
     double pixelScale = 1.0;
 #ifndef wxHAVE_DPI_INDEPENDENT_PIXELS
-    pixelScale = GetContentScaleFactor();
+    pixelScale = GetDPIScaleFactor();
 #endif
 
     m_Parser->SetDC(&dc, pixelScale, 1.0);