Fix a regression with FlxSprite's _maxFrames calculation.

The current approach assumed perfectly filled spritesheets rows. It used ceil() to adjust any floating value, which resulted in a blank column at the left of some graphics.

I've noticed that testing with a game that uses particles. Some particles had that blank column. Removing the ceil (which was Adam's original approach) fixed the problem.
Make frame "trimming" explicit by calling FlxU.floor().
Merge pull request #181 from Dovyski/fix-regression-with-FlxSprite-frames

Fix a regression with FlxSprite's _maxFrames calculation.