collectgarbage called unnecessarily in glyph_stream_provider callback

[vlasakm]

More of a question at first: Is there a reason for collectgarbage (Lua full garbage collection) to be run from the harf variant of the glyph_stream_provider callback?

I would think there's no need to perform explicit GC, and especially when run for multiple fonts back-to-back it can actually be quite slow.

Depending on whether the GC is needed for mode=harf code path, I propose to either:

1. Delete the collectgarbage call.
2. Do collectgarbage only in case the font was loaded with mode=harf.

I can provide pull request if needed, but feel free not to wait for me with such a trivial change :)

[vlasakm]

Oops, hit the button by accident before linking all the interesting context! :)

I found the GC to be the source of performance difference between luatex and luahbtex (both not using mode=harf), which was reported by @olsak:

https://tex.stackexchange.com/questions/735071/mysterious-delay-when-downloading-fonts-difference-between-luatex-luahbtex/735470#735470

I also wonder, why is the glyph_stream_provider callback seemingly only called with the TTF font? I probably got lost in all the undocumented magic constants. In any case thank you for figuring all the magic out and implementing the great HarfBuzz support!