Substantial overhaul of colour handling in do_text(). Highlights include:

Substantial overhaul of colour handling in do_text().  Highlights include:
* No more mucking about with transfer modes.  We always set up the foreground
  and background sensibly, EraseRect the whole lot and use srcOr.  Apple
  Tech Note QD505 suggests that this should be faster than srcCopy.
* Always use the Palette Manager if we've got Color QuickDraw, even in 1bpp
  modes.
* Apply the cursor colours _after_ all the other stuff, rather than trying to
  get them through everything else intact.

Unfortunately, this doesn't make much difference to the speed.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2259 cda61777-01e9-0310-a592-d414129be87e