Bug 931823 - Fix calculation of tiled update data and convert to typed units. r=kats,botond

Various changes to FrameMetrics and such have causes the tile coherency checks
to end up with nonsense values, meaning updates are always happening a tile at
a time.

This fixes the calculation of those values, adds some documentation and
converts a large part of it to using typed units, to make it easier to
understand what's happening.