This changes image size properties to be scales instead of pixel sizes.
The commit also adds fixes for handling image updates with the same ID,
adds tests for expressions in paint properties, as well as tests for using images
with pixel ratio greater than 1.
Finally it moves image-tilt-displacement and perspective-compensation properties
from layout to paint properties, and includes other minor cleanups.
This change introduces a new property type, Rotation, that uses
a custom interpolator, and that is currently applied to
all style properties named "bearing", with a period attribute.
