Multiple fixes

Includes the following changes:
* Fixes for `FlxColor` comparisons involving `null` and `Null<FlxColor>` (Thanks @mrcdk!)
* `FlxCamera` flash sprite position now updates when the camera position changes. (Previously it only ever updated during `update()`, specifically when updating the shake effect! :/)
* Using `?` instead of `Null<>` in `FlxMath.bound()` means the params need not be manually nulled.
* Added an optional means of ignoring a `FlxTouch` that has moved too much in `FlxButton`. Something like this may be useful for mouse, too, though potentially the existing mouse input isn't well suited to this change. (Note: I required this for mobile as I have implemented scroll panels that are scrolled with touch, but buttons need to be ignored if the touch that pressed them actually turns out to be a swipe to scroll the panel)
Shouldn't these setters be used in the places where we currently have `flashSprite.x = x + _flashOffset.x;` to avoid redundancy (`FlxCamera` and `CameraFrontEnd`)?