Rust upgrade
Merge pull request #111 from servo/rustup

Rust upgrade
Add a helper method for setting root layer size

This will be used by the compositor so that the top-level root layer
can track window size.
Merge pull request #112 from mrobinson/helper

Add a helper method for setting root layer size
Allow layers to mask children to their boundaries

This allows preventing iframes from overlowing their boundaries. We
also ensure that buffer requests are made for child layers, even when
their parents don't overlap the window rect. This is because children
can lie completely outside the bounds of their parents.
Allow layers to paint their own background colors

Previously there was only a scene-wide background color, but now each
layer carries its own background color. We can paint this by
generalizing the SolidLineProgram in the OpenGL renderer.