PBREmissivePass could use a different algo?

In games, often a bilateral gaussian is used rather than mipmap filtering.
Which means it could fast using stack blur rather than a mipmap pyramid.
For information, sampling the mipmaps is almost the biggest cost of the whole PBR system.

Start by locating PBREmissivePass and the PBR system's mipmap sampling path. Compare the current mipmap filtering approach with the issue's suggested bilateral Gaussian or stack blur alternatives, then establish performance and visual-quality criteria before deciding whether a change is warranted.

Written by the indexing model from the issue text.