Bug 977831. Store layer event regions on thebes layer data in appunits relative to the reference frame until we finalize the thebes layer and then convert to layer coordinates. r=roc

Event regions weren't in a workable state prior to this. They were already stored relative to the reference frame, but in layer pixels.

Inside inactive layer managers the scale is always 1 regardless of the scale on the containing thebes layer (which is the scale it will actually be drawn at), and so the event regions did not take into account scale in this situation.

Storing the regions in appunits relative to the reference frame means we can just transform to the new reference frame (if it is different) when passing event regions up and out of inactive layer managers. The proper scale will be applied when we apply the event regions to the layer.