app: prefetch entire rows of pixels when finding contiguous regions

Makes the thing about twice as fast (used by fuzzy select, bucket fill)
app: compare gamma-correced colors when finding contiguous regions

This maps threshold changes by moving the mouse more uniformly to the
growing/shrinking of the selected area, and the tools should work
perceptually anyway. Also, it makes things another 10 times as fast on
gamma-corrected drawables.