Bug 1076163 - Basic APZ support for css-driven resolution. r=kats,tn

This makes APZ behave nicely with most uses of a css transform:scale.

Summary of changes:
  - FrameMetrics::mCumulativeResolution now includes the css-driven resolution
    in addition to the pres-shell resolution.
  - Displayports are now stored in Screen pixels rather than Layer pixels.
    This is what we want anyways (as we'd like the displayport size to remain
    constant as a fraction of the screen size), but it was necessary to make
    this change as part of this patch because continuing to store them in
    Layer pixels in the presence of a css-driven resolution would have
    required a bunch of infrastructure to implement correctly.

Remaining work:
  - Layout painting a scrollable layer at a resolution different from the
    scale induced by the css transform causes problems. These will go away
    with bug 1076192.
  - Different resolutions on the x and y axes are not supported. This is
    tracked by bug 1039967.
Bug 1076163 - Clean up the resolution-related fields and methods in FrameMetrics. r=kats