fonts for BackendGL need to match functionality in svg backend

[martinjrobins]

At the moment, the BackendGL uses a single font that must be in the a hard-coded directory. Be nice if the functionality was the same as the SVG backend. I.e. can use any of the locally install fonts, or perhaps even download a web font if requested.

[martinjrobins]

note: font files in Ubuntu can be located via:

fc-list | grep "partial name here"

[martinjrobins]

for windows:

https://superuser.com/questions/1072307/where-is-my-truetypefonts-located-in-my-computer/1072308

[martinjrobins]

and mac:

https://support.apple.com/en-gb/ht201722

[martinjrobins]

So for this I need to search for a font name in the standard font directories. The names of the font files don't seem very consistent, so was thinking of searching for substrings. As for searching though directories, there is boost, or C++17 filesystem. Not keen or requiring C++17 or including something as large as boost for this. Do you know of anything else @fcooper8472?

[martinjrobins]

Using dirent.h. This is standard for linux, and I've included a windows port from here https://github.com/tronkko/dirent