Small fixes for FlxEmitter and FlxParticle

Thanks to @gamedevsam, @mrcdk, and @joecreates for pointing these out.

* Fixes behavior with `useVelocity`, `useDrag`, and `useAcceleration`
* Corrects documentation for `launchMode`
* Minor change to `FlxRange.equals()` and `FlxPointRangeBounds.destroy()`