Fix offset by one in STYLE_LOCKON camera mode.

This pull request fixes the offset by one in STYLE_LOCKON camera mode by replacing `ceil()` with `floor()` for the `targetX` and `targetY` adjustmentns. As pointed out [here](https://github.com/FlixelCommunity/flixel/pull/179#issuecomment-27531753), the use of `floor()` fixes the problem. This change does not interfere with the camera jittering fix, which remains in place and working.