Rework of NSView diesplay mechanism

[Fred Kiefer]

I just submitted a change that reworks the NSView display mechanism to
us the new method displayRectIgnoringOpacity:inContext:. To me the new
code looks a lot more logical and consistent than the old one, but as
this may only be a private opinion I would like you all to review and
test the new code.

I tried to document the display mechanism a bit better. That way you
should be able to tell, what I want to achieve with the new code and
point out errors in it. What I also would like to know is if there is
any perceivable difference in performance. I don't expect any, but you
never know.

Cheers,
Fred