Comment 2 for bug 783034

We currently have three relevant hard-wired parameters in Plot2 that could be usefully exposed:
INITIAL_PASS_MAXITER = 512 speaks for itself.
MIN_ESCAPEE_PCT = 20 // minimum % of pixels that must have escaped before we might consider this plot complete
LIVE_THRESHOLD_FRACT = 0.001 // When fewer than this fraction of the whole plot escapes in a pass, we consider it complete. When getting close to this fraction, it is doubled (and perhaps again on later passes).

These should appear in the prefs, and as command-line options.