…r=gw

Optionally serialize N frames into a circular memory buffer, and save
them as part of wr-capture into tilecache/.

The new tile_view utility reads the folder and converts the frames to
svg for easier visualization, with a few extra features:
- color code each primitive based on which slice it is on;
- highlight new or moving primitives in red (brute force diff);
- print all invalidated tiles at the top and the invalidation reason;
- overlay the tile quadtree to visualize splitting & merging;
- HTML and JS wrappers for animation playback, timeline scrubbing;

Positioning of the tiles on the screen is a bit broken still; especially
during scrolling and "special" pages (like about:config).

Interning info is not used yet.

Differential Revision: https://phabricator.services.mozilla.com/D59975

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/53cce741bb469480c121b023c94b58618e6a3355