Fix creating wxGLCanvas without any attributes in wxMSW.

This was broken by the changes of r77018, see #16402.

Just check that we do have the attributes before examining them.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77072 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Refactor Unix OpenGL code to avoid using static attributes array.

Put the context attributes in wxGLCanvasX11 itself instead.

See #16402.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77073 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775