With this patch, a minimal valid rect is calculated for each
picture cache tile. This is used to reduce the scissor rect
during tile rasterization, and the draw rect during tile
compositing, whenever there is a partial tile.

Differential Revision: https://phabricator.services.mozilla.com/D62177

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/b174d2f9072d36738d5dc484526d51a3a4f2ef82