We had a partial implementation of transformation function interpolation
that did not support numerical interpolation of simple functions (e.g.
`scale(0)` -> `scale(1)`). This refactors the interpolation to follow
the spec more closely.
Gains us 267 WPT subtest passes in `css/css-transforms`.
Fixes#6774.
The transform of each paintable was being applied multiple times due to
the recursive nature of the hit testing methods. Previously it used
combined_css_transform to transform the position, and then it would pass
that position to children, which would then apply combined_css_transform
again, and so on.
PaintableBoxes are also not hit tested anymore when having a stacking
context. A similar check is done in PaintableWithLines, but it was
missing from PaintableBox. Without this check some elements can get
returned multiple times from a hit test.
StackingContexts with zero opacity will now also get hit tested, as it
should have been before.
