Improve support for non-colour displays by adding a mask of attributes to

Improve support for non-colour displays by adding a mask of attributes to
ignore when breaking text into runs for display, and implement setting this
on Mac (other ports just use 0xffffffff).

We don't use DeviceLoop for this any more because Apple Technical Q&A
QA1024 says we shouldn't.  Unlike their example, we don't depend on the
Display Manager's being present either.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2264 cda61777-01e9-0310-a592-d414129be87e