[gsoc] webdraw - points and clipping

[David Hoskin]

So far, I have converted the graphics code to use higher-level
operations on points and rects, and then used those to implement
reliable clipping and masking. With masking working properly, I was
able to rewrite graphics functions to simply draw the required shape
on a mask, rather than hacky clipping directly on the destination
image.

Frustratingly, Acme's text selections were still not clipped properly.
Seeing that it (via libframe) uses the `d': draw() message reminded me
that I had only been working on the drawing functions themselves. It
only took a one-line change to make /dev/draw/n/ctl's `d' handler
actually use the new drawmasked(), and now Acme works perfectly!

The next step is to get rio running. I have to implement some of the
screen and windowing functions, and the sending of refresh messages.

-- David