Scan from camera demos broken in Firefox

[ttill]

Recently the "Scan … Code from Video Camera" demos stopped working in Firefox (v 67.0.2), while they still work in Chromium (v 75.0).
The error thrown after pressing "Start" is:
IndexSizeError: Index or size is negative or greater than the allowed amount originating from HTMLCanvasElementLuminanceSource:22

I have the same issue in my app since updating from 0.12.4 to 0.14.2.

[odahcam]

This is directly related to #183