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.

[originally from svn r2264]

terminal.h

@@ -155,6 +155,9 @@ struct terminal_tag {
 
     short wordness[256];
 
+    /* Mask of attributes to pay attention to when painting. */
+    unsigned long attr_mask;
+
     wchar_t *paste_buffer;
     int paste_len, paste_pos, paste_hold;
     long last_paste;