Store content offset in unscaled pixels

Content offset is now stored in unscaled pixels, so that it can be
applied to tile positions during compositor rendering without unscaling
it. This means that when using it in calculations of layer size (stored
in device pixels), we need to scale it. Scene scale is now stored as a
scale rather than a transformation.

Fixes #3269.
Fixes #3042.
Merge pull request #3318 from mrobinson/scrolling

Store content offset in unscaled pixels