restore clearing of clipped mask, turns out it is needed

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@57181 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/dcclient.cpp

@@ -1034,6 +1034,10 @@ GdkBitmap* wxWindowDCImpl::GetClippedMask(GdkBitmap* mask, int w, int h,
     c1.pixel = 1;
     wxGtkObject<GdkGC> gc(gdk_gc_new( new_mask ));
 
+    // zero-ing new_mask
+    gdk_gc_set_foreground( gc, &c0 );
+    gdk_draw_rectangle( new_mask, gc, TRUE, 0, 0, w, h );
+
     // clipping region
     gdk_gc_set_clip_region( gc, m_currentClippingRegion.GetRegion() );
     gdk_gc_set_clip_origin( gc, -x, -y );