Implement text-emphasis handling

[Gankra]

https://developer.mozilla.org/en-US/docs/Web/CSS/text-emphasis

• stylo already handles it (because gecko does)
• webrender doesn't need anything to handle it

The layout code just needs to grab the info from stylo, figure out the positioning of the glyphs, and emit a TextDisplayItem into its DisplayList.

I have marked off where text-emphasis marks should (potentially) be emitted in this PR: #17675.

[Gankra]

I guess one subtlety is that text-emphasis marks require space so you may need to adjust the whole text's baseline(?) to make room.

[njskalski]

Don't assign me yet, but I plan to give it a look.

[NeverHappened]

Hi! Can you assign me? I'm not a contributor yet though

[KiChjang]

@askalski Have you begun working on this issue? Would it be OK for @NeverHappened to start working on it?

[NeverHappened]

I've done some work on this issue and have questions. Where can I ask them? In IRC? Who should I talk to?

[jdm]

IRC or here are fine choices, especially if you don't end up receiving a response on IRC.

[NeverHappened]

1. Why do we need this feature? As I understand, only Firefox has it :)
2. As I understand, the text emphasis should be at the top and be 50% of the size of the base text. But I can't find any code that is able to scale/clone the TextDisplayItem, or easily generate new one? Does this require a lot of new code?

[jdm]

1. It's a CSS standard, and supported by Safari according to http://caniuse.com/#feat=text-emphasis .
2. @mrobinson is this a question you can answer?

[mrobinson]

@NeverHappened Any reason you need to clone the TextDisplayItem rather than just creating a new one with the properties calculated for the emphasis?

[NeverHappened]

@mrobinson No, no reason. I have issues because the code for generating display item is in the building display list phase, but the calculating text properties is done long before that.