RenderSector example: add a class to stack in the other direction, and fix various bugs.

- the circle now fills the screen
- you can set dimensions on the solid color node
- debugging printfs and rects are gone
- the protocol is changed so that for sectors, you position before you size (since your size and your childrens' positions all depend on your own position)

TBR=abarth

Review URL: https://codereview.chromium.org/1154213003