Dynamic shadows artifacts

[Deathrey (DeathreyCG)]

@Kalle-H

Created pull request for this fix. https://github.com/EpicGames/UnrealEngine/pull/4485

Edit:
So this bug is only happening when kernel is symmetric. This affect PCF2x2 and non gather 7x7 kernel.

Pretty noticeable difference for just 1 texel of a shift.