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.
At computed-value time, this is converted to whatever the parent's
computed value is. So it behaves a little like `inherit`, except that
an inherited start/end value uses the parent's start/end, which might
be different from the child's.
