Scale light shadow bias by soft_shadow_scale to reduce shadow acne

[clayjohn]

Fixes: #54523

Supercedes: #55758 and partially supercedes #55757 (this does not affect normal bias)

This PR increases the amount of bias used proportional to the radius of the PCF shadow filter. When using a larger filter radius, we need to use a greater bias. This reduces the shadow acne visible for all quality settings (because soft_shadow_scale is greater than 1 for all quality settings above hard shadows.

The unfortunate side effect of this change is an increase in peter-panning at high quality levels, but that is a necessary consequence to avoid shadow acne. I.e. without this PR if you tweak shadow_bias to the point of removing shadow acne you get about the same amount of peter-panning.

Before: Low Quality (default)

After: Low Quality

Before: Ultra Quality

After: Ultra Quality

[Calinou]

The unfortunate side effect of this change is an increase in peter-panning at high quality levels, but that is a necessary consequence to avoid shadow acne.

#55757 might help here, if we also decrease the default DirectionalLight3D shadow bias in exchange (but keep the doubled shadow normal bias from that PR).

[clayjohn]

The unfortunate side effect of this change is an increase in peter-panning at high quality levels, but that is a necessary consequence to avoid shadow acne.

#55757 might help here, if we also decrease the default DirectionalLight3D shadow bias in exchange (but keep the doubled shadow normal bias from that PR).

It is definitely worth looking into. I tried out a few variations of increased normal/decrease bias and wasn't able to reduce peter-panning much, but I think a closer look would be valuable.

[akien-mga]

Thanks!