Add attributes and properties for language and direction

[TriangulumDesire]

This pull request adds more localisation support to RmlUi, based on the discussion in #553.

To summarise, it adds the following:

• Two new internal properties (language and direction) that directly cannot be set with RCSS.
• Two new attributes for elements:
  • lang: represents the language of the element's text and sets the language property.
  • dir: represents the text-flow direction of the element's text and sets the direction property.
• A text-shaping context structure containing the language, direction, and letter-spacing, and it is used by the font engine interface.
• Unit tests for the new localisation properties.

I have created a new sample for text-shaping to show these properties in action, but I will save that for another pull request (after this one is done). Below are some images of this sample.

Font rendering without text shaping.
(English–Arabic–Hindi)

Font rendering with text shaping.
(English–Arabic–Hindi)

[mikke89]

Very cool, those screenshots in particular look excellent! Nice work.

I'm happy to see this in the library, most of these are minor and stylistic comments.

There is one performance consideration that I believe is quite important to avoid any degradation. It would be nice if you could run the Benchmarks before and after the changes in this PR, to see if we have any differences.

[TriangulumDesire]

Thanks for the feedback! I'll apply the suggestions and push a new commit sometime soon.

[TriangulumDesire]

Alright, I've applied your suggestions in another commit.

I've run the benchmarks, and it looks like there isn't much of a difference. I've attached the outputs to this comment if you would like to look at them:
before_localisation.txt
after_localisation.txt

[mikke89]

Great work! Just a couple of minor comments, then this is all good to merge in my view.

Thanks for running benchmarks too, looks good, if there are any changes then they are below the noise level.

[TriangulumDesire]

Done! I went with Get<String>. This will have a small performance penalty since it copies the string, but lang's value should be short, so copying it won't be too expensive most of the time (especially with short-string optimisation). We can always change it to retrieving a reference (with a safeguard) in the future if needed.

[mikke89]

Perfect, thanks a lot for the contribution! Looking forward to the follow-up sample :)

[TriangulumDesire]

Awesome! There's a few more tweaks I have to make to it, but it should be done within the next few days.