Fix 694579: Implement "zoom to fit page".

Add a new function pdfapp_autozoom to fit the page to the window by comparing
the aspect ratios of the page and the window to choose whether to fit
horizontally or vertically.
Set a faster default transition.
Fix 694518: Implement continuous scrolling with keyboard.
Fix 693494: Support media buttons for navigation on X11.
Fix 693517: Support /SMask/Matte preblended images.
Save a bookmark before jumping when using the 'g' command.