Fix disabling wxStaticText repeatedly in wxOSX.

Changes in r78107 would reset control's color to gray if it was disabled
more than once in a row. Guard against this and only remember the color
of an enabled control.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78144 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775