Returns the insets to apply when using this decoration on a box
that has contents, so that the contents do not overlap the edges
of the decoration. For example, if the decoration draws a frame
around its edge, the padding would return the distance by which
to inset the children so as to not overlap the frame.
Tests whether the given point, on a rectangle of a given size,
would be considered to hit the decoration or not. For example,
if the decoration only draws a circle, this function might
return true if the point was inside the circle and false
otherwise.
