Avoid using wx GDI classes from non-main thread in wxOSX/Cocoa.

Avoid using wx GDI classes from non-main thread in wxOSX/Cocoa.

OS X uses a background thread for pulsing the default button and we intercept
the draw requests from it. As our drawing code is not MT-safe, executing it
from the non-main thread can result in crashes.

Avoid this by simply not doing anything fancy when called from a background
thread and simply deferring to the superclass instead.

Closes #12407.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66041 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775