get rid of randomness in fills

Previously, each row in a fill region was offset by a random amount.  This
looks better than not offsetting at all (which will create visible columns of
stitches), but humans are very good at finding patterns in randomness.  The
result is that fill regions look weird.

This commit instead carefully orders the stitches in fill regions such that
each row's stitches are offset by half of max_stitch_length from the previous.

There are also a couple of other miscellaneous fixes and features in here --
sorry about that.

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