gloss-fluid example stutters on startup

[Ben Lippmeier]

On my 2010 QuadCore iMac, run the following:

gloss-fluid --width=400 --scale=3 +RTS -N4 -qg -qa

Interactively drawing initial density is smooth, but when I add velocity with the right-mouse button the simulation stutters for about 3 seconds before becoming smooth again. Resetting with 'r' and then drawing again yields the same behaviour.

Initialising the initial frame probably takes longer than computing one in free-running mode, and this upsets gloss's frame-rate limiting code. We should fix both problems.