expose projection

[Fil]

This is more the direction in which I'd like to go: expose just enough to make it possible to reuse the projection in another chart (we're not there yet), and to support advanced brushing (uwdata/mosaic#336).

(Not solved yet!)

We need:

• more concrete use cases (of reuse)
• tests
• enough information to recreate the same projection
• invert: currently lost in translation (^^), but we could do it by being more careful when we compose with the scale+translate transform. (The clip transform is traditionally ignored/considered as a pass-thru when inverting.)

This somewhat supersedes @jheer's suggestion in #1191 (comment)

closes #1191

[mbostock]

Should this be apply to match a scale? (And we might want an invert, too?)

[Fil]

Yes, but I was wondering if apply should be for geometries, not points. However we can handle both types since they're easy to detect (geometries have a .type property).

[mbostock]

Seems directionally good, but I don’t think it maintains the principle of the materialized projection having the same shape (or being usable) as the projection options, like we do for plot.scale(). Meaning, we want to be able to call plot.projection() and then pass the resulting object into Plot.plot as the projection option and get the same behavior. (Or maybe it does work because of the projection.stream property and I just missed it?)