Add default constructor to wxGraphicsGradientStop.

It may be convenient to create wxGraphicsGradientStop first and initialize it
later so allow doing this by providing the default argument for constructor
parameters and thus allowing to use it as the default constructor.

See #11897.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63869 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/graphics.h

@@ -309,7 +309,8 @@ extern WXDLLIMPEXP_DATA_CORE(wxGraphicsPath) wxNullGraphicsPath;
 class wxGraphicsGradientStop
 {
 public:
-    wxGraphicsGradientStop(wxColour col, float pos)
+    wxGraphicsGradientStop(wxColour col = wxTransparentColour,
+                           float pos = 0.)
         : m_col(col),
           m_pos(pos)
     {