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.
Move everything that might collect metrics to the metrics/ directory,
the only test manifests inside the render-test/ directory are the
unit tests of the probes.
