pbr: Maybe fix weird dark artifacts in PBR

[DolceTriade]

Looking at https://github.com/Nadrin/PBR/blob/master/data/shaders/glsl/pbr_fs.glsl, this repo does PBR slightly differently. In general, the calculations are similar until they come to adding it to the color. This change ported from this renderer appears to help remove black artifacts.

Before

After