consistent direction of fly picking on the fly pad

[debivort]

Flies should be picked up in same X and Y direction within each camera view as they are across the whole pad. That is, if the robot acquires first in positive Y and then in positive X, then it should pick up flies first in positive Y then positive X within each scanned region. Right now, the image tiling moves in +X, but flies are picked up in -X order. Thus, a row of flies arrayed from low to high X would not be picked up in monotonic order. Making these consistent means we can have blocks of flies loaded into the mazes in the sequence that we place them on the pad.