`DOMQuadInit` at assumes that its `DOMPointInit` members get default values

[zcorpan]

DOMQuadInit at https://drafts.fxtf.org/geometry/#dictdef-domquadinit seems to assume that its DOMPointInit members get default values. It has no provisions for those members being missing.

Originally posted by @bzbarsky in whatwg/webidl#585 (comment)

[jdm]

cc @Manishearth

[Manishearth]

Heh, made patches for this yesterday