[paint] Paint based on a paint tree instead of the layout tree

[usadson]

At the moment, the page is painted by walking the layout tree, and invoking paint calls if needed. This is fine, but having a paint tree that is constructed based on a layout tree is better for a few reasons:

1. The paint tree can be individually optimized, for example if there are two overlapping background with the same size, only the last one is needed.
2. The paint tree can be divided to compliment the tile-based rendering.
3. Scrolling the page can reuse existing tiles that are painted using these paint nodes, and new tiles can be painted without losing the complete page.

Blocks #21.