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)
>Something like this may be useful for mouse, too, though potentially the existing mouse input isn't well suited to this change.

The first touch is treated like a mouse event, which makes it important that this works for the mouse as well. Only coding input logic for the mouse is easier when you don't need multitouch in your game.