Reduce shadow acne

[Beherith]

The small negative Z bias moved the point closer to the light, so it gets less acne, and the clamping makes sure we are only shadowing areas that are more than 50% occluded. Ill try to post comparison screenshots if needed.

[rtri]

Adding an R-value bias is fine, the PCF part is personal preference (and won't do much on hardware without linear SM filtering support) best left to a custom shader.