Improved Story viewport brackets and helpers

[sdumetz]

My initial goal was to provide a visual indicator for an object's current pivot point when brackets are shown.

While I was at it I improved the style of the various light helpers. to have a more distinctive look. It should make it easier to visually identify a light type and know how it will affect the scene by looking at its viewport helper.

I ended up rewriting the brackets/helpers rendering code because I couldn't get rid of some positioning and synchronization inconsistencies otherwise. I like the new system though, it saves a render pass. I did not test whether it improved performance but it shouldn't hurt.

I took the opportunity to narrow down some property constraints where possible (mainly min: 0 where negative values wouldn't make sense).

All light types should now be usable and easy-enough to configure with the visual helpers, they just lack some UI elements to add/remove lights from a scene, for which I will have a PR coming real soon.

[gjcope]

A few issues/questions:

1. The pivot point with active brackets is easy to miss and looks to only show up where the model is not rendered. Maybe needs a different render order?
2. What's the intended use/value prop for the axes in the pose task? If it's to correlate with manual use of the transform inputs, we should probably color code those as well. Otherwise I'm not clear on how much value they are adding. I'm also seeing quite a bit of z-buffer flicker with the overlapping grid lines.
3. The directional light helpers seem to be quite small and also sometimes disconnected from the directional ray indicator (see pic below).
   

[sdumetz]

1. depends on the model's scale. Fixed in #fb9a2e02

2. Coloring the transform inputs might be a very good idea, I'll try it in the future. I made them because figuring out where we are in the pose task can be really difficult when an object is far off-center or has a weird shape. We already discussed this regarding the zoomExtentsmethod in #266 and I think this is satisfactory : It gives the user a sense of how the camera moved to make the model fit.

3. the helper's length is tied to the light's shadow near/far distances in an attempt to help users diagnose "why isn't my light casting proper shadows", which is a question I get a lot. Maybe that's not a good approach? I don't know what would be the proper size though.