Mark a transform stacking context that wraps a backdrop-filter.

A transform does not form a Backdrop Root, but WebRender still gives its
stacking context a surface when it inherits a complex clip from an ancestor.
add_backdrop_filter then chose that surface as the backdrop root, because it is
the innermost one on the stack, so the readback sourced a surface holding only
the filtered element itself and the filter had no visible effect.

WRAPS_BACKDROP_FILTER already tells WebRender to skip a stacking context when it
picks the backdrop root, so set it whenever a descendant has a backdrop-filter
and we are not forcing isolation ourselves - in which case we are the backdrop
root that the descendant should be resolving from.

Differential Revision: https://phabricator.services.mozilla.com/D322212

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=2066633
gecko-commit: 58a2d408720a3a9a1f2bfcfeb6d1aab4feabbbb5
gecko-commit-git: 91ed56fafdb5ed82503719b05604cbed4108a0d0
gecko-reviewers: layout-reviewers, emilio