Resolve warnings; signed unsigned comparison

[csaba-luxonis]

Resolved comparison between signed and unsigned.

[christian-rauch]

Instead of converting the upper bounds of the loop to a signed "int", I would prefer if you convert the running variable to an unsigned int (size_t for memory sizes) to better indicate that this running variable is a positive index.

[christian-rauch]

This is now superseded by PR https://github.com/AprilRobotics/apriltag/pull/309 where this and other warnings are fixed and reported as errors to make sure that they do not appear again.