Add `willReadFrequently` to canvas 2d context

[getify]

See this issue for further reference: nimiq/qr-scanner#236

Here, this line needs to change:

qrcodejs/qrcode.js

Line 362 in 04f46c6

this._oContext = this._elCanvas.getContext("2d");

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