Reverted to pixel-based measurements for multiple reasons.

1.  Dealing with only pixel values allows us to avoid nasty floating-point precision errors and the like beacuse we deal only in integers.

2.  No off-by-one scaling quirks when converting sprites from pixel values to unit values and back to pixel values.

3.  The "issues" I thought would be solved by using units in the first place (namely, keeping experience consistent across different resolutions) turned out to be a non-issue anyways.  In fact it is actually counter-intuitive to the goal, which I realized after the switch had been made.