Eliminate -Wcast-qual warnings with GCC and Clang

Use const_cast, mutable, and various other changes to avoid -Wcast-qual

src/osx/carbon/font.cpp

@@ -638,7 +638,7 @@ wxSize wxFont::GetPixelSize() const
 #if wxUSE_GRAPHICS_CONTEXT
     // TODO: consider caching the value
     wxGraphicsContext* dc = wxGraphicsContext::CreateFromNative((CGContextRef)NULL);
-    dc->SetFont(*(wxFont*)this, *wxBLACK);
+    dc->SetFont(*this, *wxBLACK);
     wxDouble width, height = 0;
     dc->GetTextExtent(wxT("g"), &width, &height, NULL, NULL);
     delete dc;