Implement dynamic resolution, messaging from compositor to renderer, delete unused texture layers
auto merge of #531 : eschweic/servo/master, r=metajack

There is now a SharedChan from the compositor to the renderer. Using this, tiles are re-rendered the tiles at a higher resolution after zooming (this is currently bound to a MouseClickUp event). Also, extra texture layers are deleted, which fixes a bug where old content was being displayed when a new paint message was sent.

Depends on rust-layers: 1e1e359da32c9d0ddb4f93b505a658d669008426