None of the code here actually needs a NodeWithStyleAndBoxModelMetrics,
and we'll need to be able to resolve images from inside
NodeWithStyle::apply_style().
We previously only invalidated the cached color-stop data when the
painted area's size changed. However, multiple elements can use the
same gradient and be the same size, but have different parameters that
affect the gradient stop positions, for example if a stop has an em
position. This can also change for the same element over time.
The new cache instead uses these parameters as the cache key. So we
recompute the cache if lengths would resolve differently, or the area's
size is different.
The included test fails without this change.
This reduces the number of `.cpp` files that need to be recompiled when
one of the below header files changes as follows:
Painting/Command.h: 1030 -> 61
Painting/DisplayList.h: 1030 -> 60
Painting/DisplayListRecorder.h: 557 -> 59
