Add `willReadFrequently` to canvas 2d context

[getify]

See this issue for further reference: https://github.com/nimiq/qr-scanner/issues/236

Here, this line needs to change:

https://github.com/davidshimjs/qrcodejs/blob/04f46c6a0708418cb7b96fc563eacae0fbf77674/qrcode.js#L362

The getContext("2d") would change to getContext("2d",{ willReadFrequently: true }).