Prevent re-rendering of pages because of rounding errors when computing the scale value

Currently if you reset the zoom level multiple times in a row, i.e. by pressing <kbd>Ctrl</kbd>+<kbd>0</kbd>, the pages can be re-rendered each time even though their size shouldn't change. Whether this happens can depend on the size of the viewer, but documents with pages in landscape mode seem to be very susceptible to this. (An example is: https://wiki.mozilla.org/images/5/55/MobileOpportunity.pdf.)
This can also effect documents with pages in portrait mode, when they are displayed in Presentation Mode.

The reason for this unnecessary re-rendering is that due to limited numerical precision, the last decimal place of `pageWidthScale` and `pageHeightScale` (see [pdf_viewer.js#L358-L361](https://github.com/mozilla/pdf.js/blob/master/web/pdf_viewer.js#L358-L361)) can change every time the zoom level is reset.

This patch attempts to avoid unnecessary re-rendering, by limiting the number of decimals used in the scale values.