Support Various Page Scaling Modes

[TimOliver]

At the moment, the app initially sizes all pages to fit the screen region. On smaller devices, this is sometimes not great since users will need to constantly pinch in every time they turn the page.

Other modes to consider:

• Scale to fill (Happens right now when double-tapping the screen)
• Fit to screen width (For long narrow pages)
• Fit to screen height (For wide pages)