Implement nine patch support for border gradients

[mrobinson]

This will allow border-gradients to support the full richness of the CSS
API. There are some areas where the implementation doesn't match what
CSS expects, but this should move us a good deal closer. We can
gradually fix those issues and turn this on in Gecko.

Fixes #1875.

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This change is 

[mrobinson]

r? @gw3583

[mrobinson]

This doesn't allow us to turn on WebRender border-image in Gecko yet, but gets us a little closer. We would need to investigate a bit more into why so many reference tests still fail.

[gw3583]

Reviewed 11 of 11 files at r1.
Review status: all files reviewed, 1 unresolved discussion (waiting on @mrobinson)

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webrender_api/src/display_item.rs, line 276 at r1 (raw file):

#[derive(Clone, Copy, Debug, Deserialize, PartialEq, Serialize)]
/// TODO(mrobinson): Currently only images are supported, but we will

We can remove this comment now.

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Comments from Reviewable

[gw3583]

One minor nit above, r=me otherwise. It also looks like there is no edge AA between gradient stops in the radial gradient image, but I think that's just a pre-existing issue with the radial gradient shader.

[mrobinson]

@bors-servo r=gw3583

@gw3583 Thanks for the review!

[bors-servo]

📌 Commit 54e8c80 has been approved by gw3583

[bors-servo]

⌛ Testing commit 54e8c80 with merge a185cfb...

[bors-servo]

☀️ Test successful - status-appveyor, status-taskcluster
Approved by: gw3583
Pushing a185cfb to master...

[mrobinson]

For the purposes of not breaking the build, here's a Gecko try job (should not change behavior at all) for this change: https://treeherder.mozilla.org/#/jobs?repo=try&revision=76b9810cacc13df3dc227b73026f49775815317e&selectedJob=185117848