[geometry] DOMQuad.bounds definition is unclear

It's unclear how to compute the bounds rect from the points in the DOMQuad.
We're told it's "live", and that after constructing a DOMQuad from a
DOMRectReadOnly, 'bounds' matches the rect, so quad.bounds.x = quad.p1.x.
But if someone constructs a DOMQuad({x:2},{x:0},{x:4}) then quad.bounds.x
can't be quad.p1.x (2), it must be either 0 or 4. It's not clear which one
it should be. It seems difficult to define 'bounds' in a simple way that
gives sensible results for live updates and satisfies quad.bounds.x =
quad.p1.x whenever the quad was constructed from a DOMRectReadOnly.

The simplest solution is to set quad.bounds.x to the mininum x over all
points, and quad.bounds.width to the difference between the maximum xs and
the mininum xs, dropping the requirement that 'bounds' matches the
DOMRectReadOnly used to construct a quad.

Rob
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