Create CSS mapping for tts:fontShear based on CSS skew

[cconcolato]

This is part of umbrella #406

[nigelmegitt]

I believe the proposal is to map to https://www.w3.org/TR/css-transforms-1/#typedef-transform-function using the skewX() function when the inline progression direction is the X axis and skewY() when it is the Y axis.

The 2017 CSS Snapshot explicitly allows use of the CSS3 Transforms (Level 1) spec even though it is in WD and has been since 2013, since it has widespread implementation.

[css-meeting-bot]

The Working Group just discussed Create CSS mapping for tts:fontShear based on CSS skew #471.

The full IRC log of that discussion

<nigel> Topic: Create CSS mapping for tts:fontShear based on CSS skew #471
<nigel> github: https://github.com//issues/471
<nigel> Nigel: I haven't made a change for this yet.
<nigel> .. Cyril please could you confirm my reading of this and the proposed mapping?
<nigel> Cyril: I will confirm, but I think you're right.

[css-meeting-bot]

The Working Group just discussed CSS mapping for tts:fontShear.

The full IRC log of that discussion

<nigel> Topic: CSS mapping for tts:fontShear
<nigel> github: https://github.com//issues/471
<nigel> .. Cyril, you said you would check my interpretation of tts:fontShear mapping to CSS transform skewX() and skewY() functions is correct.
<nigel> Glenn: FYI that's how I implemented it in TTPE, using SVG's skew transformations. If CSS
<nigel> s/.. Cyril/Nigel: Cyril
<nigel> Glenn: I also used rotation because certain characters in vertical mode are labelled as
<nigel> .. requiring rotation by the Unicode technical report on vertical layout.
<nigel> Nigel: Is that part of fontShear?
<nigel> Glenn: The skew functions are for fontShear, the second is for vertical layout.
<nigel> Nigel: Cyril, Glenn, if you could add a note on the issue confirming I've mapped the inline
<nigel> .. progression direction to the correct skew function that would be helpful.

[skynavga]

Fixed in 924d6cd.