Make wxRescaleCoord() private functions

They're probably not that useful in application code, which should just
use wxDPIChangedEvent::Scale() instead, so move them to a private
header instead of making them part of the public API.

include/wx/private/rescale.h

@@ -0,0 +1,43 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/private/rescale.h
+// Purpose:     Helpers for rescaling coordinates
+// Author:      Vadim Zeitlin
+// Created:     2021-07-13
+// Copyright:   (c) 2021 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_PRIVATE_RESCALE_H_
+#define _WX_PRIVATE_RESCALE_H_
+
+#include "wx/gdicmn.h"
+#include "wx/math.h"
+
+#ifdef __WINDOWS__
+    // Required in order to use wxMulDivInt32().
+    #include "wx/msw/wrapwin.h"
+#endif
+
+
+// Scale the given value by the ratio between 2 other values, with rounding.
+// Do not scale the value if it's -1, just return it unchanged in this case.
+inline int wxRescaleCoord(int n, int newScale, int oldScale)
+{
+    return n == -1 ? -1 : wxMulDivInt32(n, newScale, oldScale);
+}
+
+inline wxPoint
+wxRescaleCoord(wxPoint pt, wxSize newScale, wxSize oldScale)
+{
+    return wxPoint(wxRescaleCoord(pt.x, newScale.x, oldScale.x),
+                   wxRescaleCoord(pt.y, newScale.y, oldScale.y));
+}
+
+inline wxSize
+wxRescaleCoord(wxSize sz, wxSize newScale, wxSize oldScale)
+{
+    return wxSize(wxRescaleCoord(sz.x, newScale.x, oldScale.x),
+                  wxRescaleCoord(sz.y, newScale.y, oldScale.y));
+}
+
+#endif // _WX_PRIVATE_RESCALE_H_