Avoid scenecuts in flashes and similar situations

Avoid scenecuts in flashes and similar situations
"Flashes" are defined as any scene which lasts a very short period before a previous scene returns.
A common example of this is of course a camera flash.
Accordingly, look ahead during scenecut analysis and rule out the possibility of certain frames being scenecuts.
Also handles cases of tons of short scenes in sequence and avoids making those scenecuts as well.
Can only catch flashes of 1 frame in length with b-adapt 1.
With b-adapt 2, can catch flashes of length --bframes.
Speed cost should be negligible.