'Mark' when told to turn, turns unevenly

[compenguy]

Using the 3d-printed gamepad, when a pure turn (hard right or hard left on the analog stick) is called for, one side of the wheels turns very slowly or not at all, when the wheels are expected to run at approximately the same speed in opposite directions.

Monitoring the serial output of the gamepad during use shows that the Y-axis (up-down) scales unevenly between its positive range and negative range.

Downward scales up very quickly, while upward scales much more evenly. This is likely resulting in the case where ever-so-slightly pulling down on the stick in a turn results in a disproportionate impact on the speed, while a slight upward on the stick shows more reasonable values.

This is likely an issue of incorrectly calibrated Y axis values in Buttons.h.