In greenlight we copied this code for scanning images, either from
real image files, or from PDF files converted to images via pdf.js.

To avoid duplication clean up the internal API and expose it.

Also write some unit tests for the scan engine while at it.

This has proper support for rollup while providing the same API.

This is what open-wc recommends now and uses in their @open-wc/testing package

Use tdd in all cases to be more consisstend.
Also since last year we can use chai with rollup directly, so
do that everywhere instead of relying on karma-chai injecting
it in to the global scope.