Update to latest rust-layers

Layers now store their complete boundaries, so we can eliminate the
various ways these were stored in Servo.
Remove occlusion code from compositor

This code tried to prevent buffer requests from parts of layers that
were occluded by other layers. This doesn't really make sense when
layers can potentially be semi-opaque, and also seems to have not been
totally functional.
Remove support for hiding compositor layers

This is currently unused, because we always know the size of the layer.
Remove page_size member from CompositorData

The page size is always the same as the layer boundaries since,
SetLayerClipRect was always called after layer creation.
Update to latest rust-layers
Merge pull request #2798 from mrobinson/more-rust-layers-cleanup

More rust layers cleanup