Render in parallel

[brson]

This round-robins tile rendering across cores. I put this together pretty quickly at the end of the work week, but it's probably close to ready. The speedups aren't as good as you might expect (perf-rainbow is decent), but it is generally faster, these test cases aren't all that rendering-intense, and there's probably some unknown synchronization happening deeper in the stack.

Some numbers (in ms) on my 4 core machine:

CPU mode, en.wikipedia.org/wiki/Rust

before: RenderingCategory: 108.3725 102.9995 2.7500 280.4812 91
after: RenderingCategory: 85.5513 87.8209 3.6918 149.5089 121

GPU mode, en.wikipedia.org/wiki/Rust

before: RenderingCategory: 383.4462 308.6807 151.7053 840.7829 22
after: RenderingCategory: 338.4899 360.0376 90.4928 586.7247 54

CPU mode, lipsum.html

before: RenderingCategory: 43.8702 41.5389 0.4079 119.9315 165
after: RenderingCategory: 51.1924 48.0576 0.3047 123.5846 113

GPU mode, lipsum.html

before: RenderingCategory: 374.0989 396.5835 88.6352 629.7278 30
after: RenderingCategory: 222.2546 229.1403 58.1653 442.7781 46

CPU, perf-rainbow.html

before: RenderingCategory: 54.5183 56.3723 29.4359 93.4502 22
after: RenderingCategory: 25.5713 22.3374 2.6982 93.4121 65

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[highfive]

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[pcwalton]

We have a layout_threads option in gfx::opts for this; I would suggest just using the same value.

[brson]

I actually found an n_render_threads value in that same struct that I wired up in a follow up. Do you think I should combine them or leave the separate knobs?

[brson]

The most recent push of this branch will definitely not pass tests quite yet.

[brson]

Passes make check on linux now.

[pcwalton]

So $10 says what's happening is that the glyph cache is per-thread and much of the time is spent filling it up, reducing the parallel gains. I can profile tomorrow to check to see if that's the case.

[brson]

Rebased.