Adding a native implementation for clearing bitmap/window contexts

Filling a bitmap surface is filled with ARGB 0,0,0,0. This way eg buffered transparent layers can be properly cleared.

src/common/graphcmn.cpp

@@ -759,6 +759,11 @@ void wxGraphicsContext::DrawRectangle( wxDouble x, wxDouble y, wxDouble w, wxDou
     DrawPath( path );
 }
 
+void wxGraphicsContext::ClearRectangle( wxDouble WXUNUSED(x), wxDouble WXUNUSED(y), wxDouble WXUNUSED(w), wxDouble WXUNUSED(h))
+{
+    
+}
+
 void wxGraphicsContext::DrawEllipse( wxDouble x, wxDouble y, wxDouble w, wxDouble h)
 {
     wxGraphicsPath path = CreatePath();