Canvas: implement context state save/restore.

[mmatyas]

This patch enables the use of save() and restore() for the canvas context, which is used by a lot of sites and scripts.

Depends on servo/rust-azure#153.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/4710

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[jdm]

Reviewed files:

• components/canvas/canvas_paint_task.rs @ r1
• components/script/dom/canvasrenderingcontext2d.rs @ r2
• components/script/dom/webidls/CanvasRenderingContext2D.webidl @ r1
• tests/wpt/metadata/2dcontext/line-styles/2d.line.width.transformed.html.ini @ r1
• tests/wpt/metadata/2dcontext/the-canvas-state/2d.state.saverestore.bitmap.html.ini @ r1
• tests/wpt/metadata/2dcontext/the-canvas-state/2d.state.saverestore.fillStyle.html.ini @ r1
• tests/wpt/metadata/2dcontext/the-canvas-state/2d.state.saverestore.globalAlpha.html.ini @ r1
• tests/wpt/metadata/2dcontext/the-canvas-state/2d.state.saverestore.lineCap.html.ini @ r1
• tests/wpt/metadata/2dcontext/the-canvas-state/2d.state.saverestore.lineJoin.html.ini @ r1
• tests/wpt/metadata/2dcontext/the-canvas-state/2d.state.saverestore.lineWidth.html.ini @ r1
• tests/wpt/metadata/2dcontext/the-canvas-state/2d.state.saverestore.miterLimit.html.ini @ r1
• tests/wpt/metadata/2dcontext/the-canvas-state/2d.state.saverestore.stack.html.ini @ r1
• tests/wpt/metadata/2dcontext/the-canvas-state/2d.state.saverestore.stackdepth.html.ini @ r1
• tests/wpt/metadata/2dcontext/the-canvas-state/2d.state.saverestore.strokeStyle.html.ini @ r1
• tests/wpt/metadata/2dcontext/the-canvas-state/2d.state.saverestore.transformation.html.ini @ r1
• tests/wpt/metadata/2dcontext/the-canvas-state/2d.state.saverestore.underflow.html.ini @ r1
• tests/wpt/metadata/2dcontext/transformations/2d.transformation.scale.negative.html.ini @ r1
• tests/wpt/metadata/html/dom/interfaces.html.ini @ r1

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^{components/script/dom/canvasrenderingcontext2d.rs, line 347 [r2] (raw file):}
Where's this defined?

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^{components/script/dom/canvasrenderingcontext2d.rs, line 387 [r2] (raw file):}
nit: indentation

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^{components/script/dom/canvasrenderingcontext2d.rs, line 885 [r2] (raw file):}
Why &?

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^{components/script/dom/canvasrenderingcontext2d.rs, line 901 [r2] (raw file):}
Why &?

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^{components/script/dom/canvasrenderingcontext2d.rs, line 919 [r2] (raw file):}
Why &?

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^{components/script/dom/canvasrenderingcontext2d.rs, line 937 [r2] (raw file):}
Why &?

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Comments from the review on Reviewable.io

[mmatyas]

^{components/script/dom/canvasrenderingcontext2d.rs, line 347 [r2] (raw file):}
This line sets the current state to the last one in the stack. Do you know a better way to do this?

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^{components/script/dom/canvasrenderingcontext2d.rs, line 885 [r2] (raw file):}
Whoops, fixed.

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Comments from the review on Reviewable.io

[jdm]

Reviewed files:

• components/script/dom/canvasrenderingcontext2d.rs @ r3

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^{components/script/dom/canvasrenderingcontext2d.rs, line 347 [r2] (raw file):}
I mean where is clone_from defined? I don't see it anywhere in this pull request.

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Comments from the review on Reviewable.io

[mmatyas]

^{components/script/dom/canvasrenderingcontext2d.rs, line 347 [r2] (raw file):}
Ah, that comes with #[derive(Clone)], at the definition of struct CanvasContextState.

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Comments from the review on Reviewable.io

[jdm]

Ok! Go ahead and squash, and use the appropriate update-cargo command to grab the rust-azure changes.

[mmatyas]

Ok, squashed this and servo/rust-azure#153 too, I'll update the lock files as soon as it gets merged.

[mmatyas]

Updated & rebased.

[jdm]

@bors-servo: r+

[bors-servo]

📌 Commit 5b8416a has been approved by jdm

[bors-servo]

⌛ Testing commit 5b8416a with merge 9c7c289...

[bors-servo]

☀️ Test successful - android, gonk, linux1, linux2, mac1, mac2