Attribute masking implemented. The variable attr_mask stored the current

set of "interesting" attributes, and do_paint only takes those ones into
account when deciding when to break style runs.  Makes redraw quite a bit
faster on displays which don't do ANSI colour.

[originally from svn r102]

putty.h

@@ -70,6 +70,8 @@ typedef HDC Context;
 #define GLOBAL extern
 #endif
 
+GLOBAL unsigned long attr_mask;
+
 GLOBAL int rows, cols, savelines;
 
 GLOBAL int font_width, font_height;