Don't scale the value(s) of -1 in wxWindow::FromDIP().

Using FromDIP() in wxXRC broke creating controls whose width or height was
specified as -1 as it became -2 when sufficiently high DPI was used, and so
lost the special meaning of "unspecified" that -1 had.

Avoid this problem by never scaling -1 in FromDIP(), this is unlikely to ever
be useful and could result in more difficult to debug problems in the future.

src/common/wincmn.cpp

@@ -2873,8 +2873,10 @@ wxWindowBase::FromDIP(const wxSize& sz, const wxWindowBase* WXUNUSED(w))
 {
     const wxSize dpi = wxScreenDC().GetPPI();
 
-    return wxSize(wxMulDivInt32(sz.x, dpi.x, BASELINE_DPI),
-                  wxMulDivInt32(sz.y, dpi.y, BASELINE_DPI));
+    // Take care to not scale -1 because it has a special meaning of
+    // "unspecified" which should be preserved.
+    return wxSize(sz.x == -1 ? -1 : wxMulDivInt32(sz.x, dpi.x, BASELINE_DPI),
+                  sz.y == -1 ? -1 : wxMulDivInt32(sz.y, dpi.y, BASELINE_DPI));
 }
 
 #endif // !wxHAVE_DPI_INDEPENDENT_PIXELS