Fix layer / stacking context namining inconsistencies

[glennw]

The use of layer as a term is inconsistent between the cpu and gpu portion of webrender. Very confusing...

[kvark]

Could be seen as a sub-task of #482

[nical]

I propose using the "layer" terminology for the moving things that webrender bases its internal transforms on (like layers::Layer), and consistently use "stacking context" for things that map "1 to 1" to actual css stacking contexts.
Layers would be only scroll roots today, but if/when we are to support async animations and other fancy features, such elements would probably generate layers as well.

I'd appreciate if we wait for PR #568 to land before writing code for this to avoid ugly merge conflicts.

[glennw]

This can be considered complete with the removal of ClipScrollGroup and PackedLayer.