Improve QR code scanning for cable_tunnel example

[micolous]

This issue is free to pick up if you're interested; this is mainly a brain dump to track things. This work is part of #259.

The caBLE authenticator implementation example currently needs a may to get FIDO:/ URLs from caBLE QR codes into it. It currently has two mechanisms to supply this:

1. inputting the URL directly
2. scanning an image for a QR code (using bardecoder)

I like having the scanning feature available, because it made things pretty convenient during testing and development. My workflow there was to:

1. start the authentication flow on the device
2. take a screenshot of the QR code, either using developer tools (for a mobile device) or local screenshot tools
3. save the screenshot to a file
4. open the screenshot with cable_tunnel

This is currently about 80% reliable after manual cropping (which I ideally don't want to do!). There's currently a hack in it to make it accept QR codes from iOS initiators more reliably:

https://github.com/kanidm/webauthn-rs/blob/e9ebff3123263073c3668228fa088f007f3fa555/webauthn-authenticator-rs/examples/cable_tunnel/core.rs#L173-L174

Some observations:

• iOS caBLE QR codes have a small logo over the centre of the QR code, and no custom pixel styling; the colours are not black and white either so needs the contrast tweak.
• Chrome (on desktop) caBLE QR codes have a large logo over the centre of the QR code, circular pixel styling, but black and white colours. The contrast tweak seems to have no effect: https://github.com/piderman314/bardecoder/issues/46

I'm able to reliably scan these QR codes with zxing and Google Camera on Android, and with iOS camera ~100% reliably; and that's taking a picture of the screen, rather than the pixel-perfect conditions of a screenshot. They can cheat a little by using multiple frames in the case of a scan failure, but the process feels very responsive.

Work ideas here include:

• working with bardecoder to figure out if there's ways that algorithm can be improved
• investigating alternative libraries (eg: rxing, zbar / zbar-rust) to see if they work better

I'd like to be able to use a device's webcam to scan QR codes in this demo, but this can't really happen until the QR code scanning is more reliable and faster.

[micolous]

Windows 11's built-in caBLE support uses a QR code with square pixels and a logo over the centre (white square with black logo, similar to Chrome). These are also unreliable to scan from screenshots (well, a HDMI capture device), and the only way around it is to re-roll the QR code (by aborting the caBLE transaction).

The logos on Chrome, iOS and Windows are all of a similar size.

I suspect the design of the logo itself is triggering the issue, but as before, using other scanners has been reliable.