A series of Layer <-> RenderingContext refactoring.

- phase 1: simplifying Layer._render.
- phase 2: clearRect and beginEyeRender.
- phase 3: small pieces of refactoring.
- phase 4: RenderingContext does actual Layer.render.
- phase 5: refactors a little bit more.

phase 6: Removes RenderingContext.fillStyle property access.
Since it is only used in DEBUG mode. Also it has prevented Tombo.js/sample not to be compiled because it depended on util.jsx under marica.
Confirmed now standalone-Tombo.js works fine.