Retina Hidpi display

Hi,
That's great. But it doesn't work at best on mobile devices or generally hidpi screens.

I have found that if I change a value in the viewport definition
` var viewport = page.getViewport(domMapObject.canvas.width / page.getViewport(0.5).width);`
instead of
`var viewport = page.getViewport(domMapObject.canvas.width / page.getViewport(1).width);`
on line 818 of app.js

make it work at retina resolution

But it works only at first. when you change page and revert back, the page is left black.

I can't find what happen when it go back to a page already drawn and why it left it all black.

How could I fix this ?
Where the viewport value is reset in some way ?

Thanks

Start in app.js around line 818 and reproduce the issue on a Retina or other HiDPI display by changing pages and returning to an already drawn page. Trace how the viewport value and canvas state are handled; done means pages render at the intended resolution without becoming black after navigation.

Written by the indexing model from the issue text.