Bug 1559535 - Fix crash related to mismatched clips with picture caching. r=kvark
Bug 1559535 - Fix crash related to mismatched clips with picture caching. r=kvark

Fixes an edge case where splitting the top level primitive list
for picture caching may result in a mismatched push/pop clip
pair.

This is not a particularly efficient fix, but it's a rare enough
edge case for now that this fix will be good enough until we work
out the long term solution for the push/pop clip chain instances.

Differential Revision: https://phabricator.services.mozilla.com/D35139

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/11f0e1ee8f0c9ec82c81ed842fc68402429d4441

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender