Fix getPolygonOffset prop

[Pessimistress]

polygonOffset parameter was not applied.

[ibgreen]

The original idea was that we would forward the settings to the layer's draw function, but we can certainly apply them here.

We wanted to support props.settings, this is a nice way to do it. Won't allow overriding the layers hardcoded draw settings though, but maybe that is a good thing.

Not sure if we are actually supplying props.settings somewhere, maybe they are already supplied by layer-manager or draw-and-pick?)

[Pessimistress]

polygonOffset was a feature that layers used to get for free. Do we have a good reason to ask layers to do more work in their draw methods now?

[Pessimistress]

Also settings is not documented - do we have a good use case for it? If yes, maybe should call it glParameters to be clear?

[ibgreen]

There should be many use cases for a layer settings prop (props.parameters or props.glParameters). An editing app that I work on now wants to control blend modes per layer, and this parameter would handle that nicely with our stock layers.

Exposing this gives a ton of flexibility to apps, at a very low API cost for deck.gl (luma.gl already does the heavy lifting of allowing parameters to be specified as keys in objects).

No objections to changing to glParameters although it would be (partly) inconsistent with luma.gl, where the the new Model.draw({uniforms, ..., settings}) function takes a settings field. That said, the global luma.gl function is called setParameters.

I'd probably vote for parameters and if so, maybe update Model.draw({settings}) to take Model.draw({parameters}) as well?

@1chandu @shaojingli