WebCanvas: `enableDevicePixelRatio` does not work in web worker

[zubiden]

Since devicePixelRatio is not defined for Web Workers, it results in an error when enabled.

I suggest removing this option and providing a new one that accepts a scale factor number. If a number is not provided, it should fall back to the current value of 1 + ((window.devicePixelRatio - 1) * 0.75). In environments where DPR is not defined, it is simply 1. I would name it something like scaleFactor, renderScale or canvasScale (debatable).

[tinyjin]

@zubiden Web Worker runs in independant context, which doesn't contain window accessor.
Please note, this package doesn't support web worker based rendering yet. We might be planning this further

At the moment, by toggling of enableDevicePixelRatio to false, the error from accessing window.devicePixelRatio will be gone. (When it's false, it doesn't access to devicePixelRatio.

We'll revisit for web worker support later. Thanks