client{X,Y}->offset{X,Y}
client{X,Y}->offset{X,Y}

Currently we rely on the fact that the webgl view is full screen when
calculating mouse interactions. by moving to offset, we take a stronger
dep on jquery, but now the webgl view can be anywhere. useful towards
the widget too.

menpo/landmarker.io,menpo/landmarker.io,jabooth/landmarker.io,menpo/landmarker.io,jabooth/landmarker.io