Implement wxWindow::GetTextExtent to use wxDC to at least return something

Implement wxWindow::GetTextExtent to use wxDC to at least return something
reasonable rather than leaving uninitialized garbage values unchanged.

Fix wxWindow::DoSetVirtualSize so that the underlying Cocoa view always
takes up at least as much area as its enclosing clip view.  In particular,
wxDefaultCoord was not being handled, resulting in -1 size components.

Together, these make the generic wxListCtrl mostly usable.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@49137 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775