Improve viewer resolution

[lfoppiano]

This PR attempts to improve the PDF resolution in the viewer, that, even if we are following the solution found in many places, it's not satisfying.

We introduced a variable called resolution_boost that apply a boost on how the resolution is calculated. The value goes from 2 to 10, with default to 1.
The canvas of the PDF pages is built by getting the Viewport (unscaled) and calculating the scaling ratio by dividing the expected width and the actual viewport width.

When the canvas is created, we use the scaled viewport dimensions and we multiply by a ratio value due to the density of pixels of the device (given by window.devicePixelRatio), in the current version, this value is either 1 or 2 (retina displays).

However, the quality of the PDF shown still is not optimal.
So we added a boost ratio that will be a multiplied for the calculated ratio:

      const ratio = (window.devicePixelRatio || 1) * resolutionRatioBoost

this approach, gave acceptable results in our experiments. Nevertheless, we did not find any explanation and it might be used incorrectly.

Higher ratio is definitely more resource-demanding on the browser, so use carefully, and, at your own risk :-)