Make QRcode decoding non-blocking by doing it in a thread

[Cheaterman]

I noticed my entire application was frozen the first time I used zbarcam, this seems to fix it. 👍

[AndreMiras]

Thanks @Cheaterman for giving it a try.
It looks OK on the diff even though I haven't tried.
Note the unit tests are failing, could you try to update your PR fixing them.
The error is:

>       symbols = self.zbarcam._detect_qrcode_frame(texture, code_types)
E       TypeError: _detect_qrcode_frame() missing 1 required positional argument: 'code_types'

refs https://travis-ci.com/github/kivy-garden/zbarcam/jobs/371384314

[Cheaterman]

Thanks for taking a look @AndreMiras - lemme fix that (I changed the signature of _detect_qrcode_frame())

[Cheaterman]

I think that will do :-)

[Cheaterman]

Welp - this should be better. I had things set up in a way that wouldn't work if _on_camera_ready was never called - and it's not called in tests.

[Cheaterman]

Haha, I'm silly, the tests need to run differently because of the now asynchronous nature of _detect_qrcode_frame() - I'll rewrite my code differently instead, leaving the function as it is, and using it inside of a separate threaded function.

[Cheaterman]

It's "more or less" left as-is, since I need to pass the pixels separately (can't write to texture.pixels) ; I made it an optional kwarg and it uses texture.pixels by default.