Allow to use FreeTypeAbstraction fonts and render glyph by index

[Kolaru]

These are small features that I am porting from CairoMakie so that they will be available to Luxor as well, to allow support for MathTeXEngine v0.5.

I do not know if I should add tests for those or what those tests should look like.

[cormullion]

@lobingera How does this look to you?

[lobingera]

Hmm, I lost track how we do things in julia today. Code looks reasonable and i think i have a local branch that does more or less the same, but i never saw libfreetext working correctly on all platforms.

Testing - just calling all functions - would be nice. Having a standard font available that loads in all platforms is a plus. Doing an example is the highest i can ask for.

[davibarreira]

This PR is greatly appreciated!

[ViralBShah]

Please let me know if this should be merged? Also happy to give access to a couple of more folks to maintain the package.

[cormullion]

Please let me know if this should be merged? Also happy to give access to a couple of more folks to maintain the package.

It would be great if Cairo.jl has occasional maintainers - it's pretty stable and unchanging generally.

I think Cairo.jl is still fairly central to the Julia ecosystem - probably needs releases and updates occasionally.

This PR probably needs some tests...

[ViralBShah]

Good point. Maybe we should see if any of the folks who have made a couple of PRs to Cairo.jl are interested in stepping up for light maintenance.