gfx: Update Azure and Skia, and rewrite broken clipping logic.

This exposed some problems in our clipping logic, which was never
properly rewritten for the stacking context reform. The clipping code
worked in terms of a stack of clips, but the new stacking context code
has no concept of a stack of clip regions. Fixing that in turn exposed
some flaky/incorrect tests:

* `borders` had an incorrect reference image, as far as I can tell.

* `negative_margins` had some stray pixels, fixed by changing the text.
auto merge of #4272 : pcwalton/servo/upgrade-azure, r=mrobinson

This exposed some problems in our clipping logic, which was never
properly rewritten for the stacking context reform. The clipping code
worked in terms of a stack of clips, but the new stacking context code
has no concept of a stack of clip regions. Fixing that in turn exposed
some flaky/incorrect tests:

* `borders` had an incorrect reference image, as far as I can tell.

* `negative_margins` had some stray pixels, fixed by changing the text.

r? @mrobinson