1. Provide the same API, especially, but not limited to, wxGLCanvas and
wxCLContext ctors (which were completely different in all ports)
2. Extracted common parts into wxGLCanvas/ContextBase classes
3. Deprecate the old API using implicitly created wxGLContext
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@45388 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775