Fixed up relative positioning so it no longer drifts due to rounding-bias errors when flipping horizontally.  This has been done by actually storing the relative-x/y values rather than deriving them from the difference.  Because we're just setting the child's position directly every frame, this also greatly simplifies the logic for this.  This does break the few usages we have of relative-positioning, such as frogatto's head/tongue.
Merge branch 'master' of github.com:anura-engine/anura