internals: gesture graphics improvements

* the gesture draw command uses a boolean to say whether it's drawing for picking reasons or for rendering reasons. This could use an enum to be more obvious.
* the thick-line drawing using strips currently puts all strips into the same list even if they are going to be drawn in different layers. So when we loop over the layers, each layer has to iterate over all the strips. A bit of extra infrastructure could let the strips be organized per-layer.