Fix the PDF rendering quality

Fixes #369

The problem we were hitting was because we were setting the viewport scale
to 1 when rendering each of the PDF pages. As I was digging into this, I
found this page, which describes exactly what's going on:

http://stackoverflow.com/questions/35400722/pdf-image-quality-is-bad-using-pdf-js

Basically, this is how it breaks down.  We create a canvas element to render the
PDF page onto, and it is set to the viewport height and width. Because
many (most?) PDFs are rendered at a smaller resolution, what ends up
happening is on higher resolution monitors we create a big canvas, but then
render an image that isn't scaled to it right.  This causes the pixels to
get spread out and blurriness ensues.

The fix is to set the scaling to "2".  We played around with the 3-5 range
and it looked good on high resolutions - but as soon as the same image
was rendered on a smaller resolution screen things went south quick.  2
seems to be the right setting which straddles the fence between the bad
and pretty good.

Author: leakymemory

src/scripts/contentCapture/pdfJsDocument.ts

@@ -48,7 +48,13 @@ export class PdfJsDocument implements PdfDocument {
 		return new Promise<string>((resolve) => {
 			// In pdf.js, indexes start at 1
 			this.pdf.getPage(pageIndex + 1).then((page) => {
-				let viewport = page.getViewport(1 /* scale */);
+				// Issue #369: When the scale is set to 1, we end up with poor quality images
+				// on high resolution machines. The best explanation I found for this was in
+				// this article: http://stackoverflow.com/questions/35400722/pdf-image-quality-is-bad-using-pdf-js
+				// Note that we played around with setting the scale from 3-5, which looked better on high
+				// resolution monitors - but did not look good at all at lower resolutions. scale=2 seems
+				// to be the happy medium.
+				let viewport = page.getViewport(2 /* scale */);
 				let canvas = document.createElement("canvas") as HTMLCanvasElement;
 				let context = canvas.getContext("2d");
 				canvas.height = viewport.height;