Minor logic fix for Sprite life update

The subtraction of `physicsElapsedMS` needs to be done for all individual
updates. (When current FPS ~ target FPS this is a 1-1 mapping, but catch-up
updates can throw off the calculations.)

Also renamed `Game#updateNumber` (a poor initial name on my part) to
`currentUpdateID`. This matches the naming of
`Stage#currentRenderOrderID`.
This is the actual logic fix.