Implement wxMask copy ctor for wxGTK.

Without copy ctor copying masks simply crashed because the same pointer was
deleted twice.

Also added a (completely trivial but better than nothing...) unit test for
wxBitmap to check that copying masks does work now.

Closes #11854.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63774 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775