VUI: Scrolling/panning

[kpreid]

When there is large content to display (or the window is a funny size), the UI must allow scrolling.

• VUI view state includes a mutable camera position, and vui::PageLayout::view_transform() computes not just a single position but bounds within which it can move.
• The camera position can be modified by one or more of:
  • scroll events (if a scroll wheel/touchpad is available)
  • touch drags
  • movement keys (if not already taken over by some other function like text input)