Fix scaling and display high PDF in high resolution

[lfoppiano]

This PR addresses #48, providing a better resolution.

In addition:

• the desired width should never be larger than the available screen, so it does not show half PDF in the screen
• the width scales also when the window is smaller than the desired with

So far results looks better:

Some references: https://gist.github.com/callumlocke/cc258a193839691f60dd