Auto merge of #19141 - upsuper:window-transform, r=emilio

Implement -moz-window-transform{,-origin} internal properties

This fixes [bug 1374178](https://bugzilla.mozilla.org/show_bug.cgi?id=1374178).

Note that the impl of `-moz-window-transform-origin` doesn't exactly match how it is implemented in Gecko. Specifically, that property doesn't accept depth value in Gecko, but it does in this impl. The depth value is simply dropped during conversion. This is because I don't think it's worth adding code for handling this internal property to make it as restrictive.

This is a modified version of #19106. The only difference since then is how `impl_transform_origin` distinguish between `transform-origin` and `-moz-window-transform-origin`. The previous PR has a bug which checks against `transform` rather than `transform-origin`, while this PR changes it to checking the length of the array. It introduces an unsuppressible warning due to rust-lang/rust#45850. This doesn't cause build failure, so maybe it's fine?

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