relaxing min length of corner marks needing to be required.

lowering the corner_mark_min_length allows us to accept mis-scaled and more crooked corner markings.  This change also adds debugging to the image.c calculate_matrix that was previously just returning NULL because it couldnt find at least 3 corners.  Adding this error will automatically be caught by the sdaps_client log parser.

https://github.com/3dna/megatron/pull/35 will not pass until these changes have been merged.


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