[WIP] Add spatial clustering to pictures.

[gw3583]

This improves performance by reducing the amount of per-primitive
work that is required each frame. It also groups primitives that
are near each other, which will be helpful for efficiently
determining which parts of a cached picture are valid.

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This change is 

[gw3583]

This is a [WIP] for now, because:

• There are a couple of test failures on try.
• It (very slightly) regresses performance on dl_mutate on talos, which is far from ideal!

I'll investigate these tomorrow, but posting for now in case anyone has comments on the general approach.

[gw3583]

@pcwalton Not a big deal, but seeing this on CI:

./Cargo.lock:1: duplicate versions for package `bit-vec`
	The following packages depend on version 0.4.4 from 'crates.io':
		pathfinder_partitioner
	The following packages depend on version 0.5.0 from 'crates.io':
		webrender

Would it be possible to get pathfinder_partitioner updated to use the newer bit-vec and published?

[pcwalton]

@gw3583 On it.

[pcwalton]

@gw3583 pcwalton/pathfinder@5db0ecf

[gw3583]

Closing in favor of landing this in parts, starting with #3313.