Added code to track and report last paint delta.
Added code to track and report last paint delta.

This is useful for animation APIs that just want a delta, or that want to
accumulate time. It saves them from having to initialize a start time and then
subtract the current time from that start time.

longtaoge/playn,tinkerstudent/playn,tinkerstudent/playn,playn/playn,rachelharvey/playn,playn/playn,playn/playn,thecocce/playn-1,longtaoge/playn,rachelharvey/playn,ruslansennov/playn2,thecocce/playn-1,ruslansennov/playn2