…curacy. r=nical,Bert

This patch introduces a new reftest (syntax ** or !* in reftest files).

This type of test renders a single input file multiple times, at a range
of different picture cache tile sizes. It then verifies that each of the
images matches (or doesn't).

This can be used to verify rasterizer accuracy when drawing primitives
with different tile sizes and/or dirty rect update strategies.

One of the included tests in this patch fails the accuracy test - the
intent is to fix this inaccuracy in a follow up patch and then be able to
mark it pixel exact.

Differential Revision: https://phabricator.services.mozilla.com/D60185

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/918e156c75aa1ac1fbfdd0a81434c68eb9d11f18