expose text measurement during layout phase

[cmyr]

Currently text measurement is only possible once a handle to the RenderCtx is available, which is only in the paint fn. To do layout correctly, we need to be able to measure text in the layout fn. Not sure exactly how this should be plumbed, but it is currently preventing us from properly implementing most text-focused widgets.

[SimonSapin]

Would linebender/piet#47 affect this?

[raphlinus]

Yes, these overlap. We need some Rust type other than a piet RenderCtx that's capable of doing text layout. Ideally you'd be able to take the text layout object from that and draw it in a RenderCtx. As in piet#47, we need to figure out what operations this is capable of. We also need to figure out whether this should simply be part of piet, or split out into a separate crate, as it's not hard to imagine there's value in doing text formatting without it being tied to piet drawing.

[raphlinus]

Close this? There are a whole bunch of text improvements in the pipeline, but I think the core issue here has been addressed.