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
Make send_buffer_requests_recursively batch messages

Also, rename it to get_buffer_requests_recursively to reflect that the
caller has to send all of the batched messages itself.
Separate the contents of ReRenderMsg into its own type

Make a ReRenderRequest type to represent the contents of a ReRenderMsg.
Currently, ReRenderMsg just takes a single one, but it in the future it
will take multiple requests.
Batch ReRenderRequests rather than render_task::Msg

Make get_buffer_requests_recursively create a ReRenderRequest rather
than a ReRenderMsg, and have its caller create the ReRenderMsg from
the ReRenderRequest.
Group ReRenderRequests by PipelineId
Only clone the RenderChan once per Pipeline
Make ReRenderMsg take multiple ReRenderRequests
Check whether the render task has permission to paint before painting

This matches the behavior of RenderMsg. It doesn't make sense to go to
the trouble of rendering if the render task doesn't have permission to
paint in the first place.
Batch replies in RenderTask::render
Make RenderListener::paint take multiple replies
Make the CompositorTask Paint message take multiple replies
Remove FIXMEs related to batching buffer requests