Import images as planes: use Principled BSDF for emission mode

Since Blender 2.91, the Principled BSDF node has an Emission Strength
input. It can thus replace the previous setup used to import images as
emissive planes. This can improve export compatibility.

Reviewed By: Bastien Montagne (mont29)

Differential revision: https://developer.blender.org/D9669