Move shadow rect computation, offset translation, and corner radii
spread-distance adjustment from the Skia display list player to the
recording site in ShadowPainting.cpp. This avoids recomputing derived
geometry every time the display list is replayed.
Replace PaintBoxShadowParams with precomputed fields directly on
PaintOuterBoxShadow and PaintInnerBoxShadow command structs. Add
CornerRadii::adjust_corners_for_spread_distance() to centralize the
CSS spec's spread-distance-to-radius adjustment logic.
PaintContext dates back to a time when display lists didn't exist and it
truly represented "paint context". Renaming it to better align with its
current role.
In the upcoming change, device pixel conversion of ClipFrame will
happen during display list replay, where PaintContext is not available,
so let’s move it out of PaintContext.
