Clipping for RECT types. Breaks a little.

When using QuadTrees, RECT_OUTLINE and RECT_FILLED are clipped to
the quadtree node.

This lets you zoom in forever on rectangles with a couple of caveats:
1. This is technically wrong for RECT_OUTLINE, as more lines will be added.
2. There are a couple of issues related to the fact that only one node is
   visible at a time. Notably:
   (a) You lose the infinite precision when stradling a quadtree boundary,
       which will happen eventually.
   (b) Zooming out will show random broken things.

However, if you look at shape.svg, you can see how moving the camera around when
both the RECT and the BEZIER are in view, the RECT moves smoothly and the BEZIER
doesn't.