pause FlxFlicker

[Geokureli]

Add pause and resume to FlxFlicker, I saw it in the Funkin branch

[EliteMasterEric]

For context with regards to the double release bug, I encountered that when I wanted to make a slower flicker, followed by a faster flicker, then stop flickering.

However, calling FlxFlicker in the onComplete callback would destroy any existing flickers on the object, then recycle that destroyed flicker, but the recycled flicker was the one currently calling onComplete so it would immediately call release on the flicker that was supposed to be active.

[Geokureli]

Note: I added FlxTween.flicker, in #3086, which offsers pausing and resuming but I may still add them to FlxFlicker