Auto merge of #1796 - staktrace:explicit_parents, r=glennw
Auto merge of #1796 - staktrace:explicit_parents, r=glennw

Allow webrender_api callers to explicitly specify the parent when def…

…ining a new clip or scroll frame.

This will allow us to use these APIs with less pushing and popping of
ancestor clips onto the stack.

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