Metallic and Roughness workflow

[PlasmaDev5]

Many other renderers that use PBR look to work on a Metallic and Roughness workflow, i feel adding this to Sparky could be useful for people coming from these engines along with many assets being made for Metallic and Roughness workflows.

P.S im happy to look into implementing.

[TheCherno]

Yes, supporting metalness (instead of specular) would be nice from a compatibility point of view. It will need a slightly different shader though, so feel free to look into that if you like.

[PlasmaDev5]

i ran into issues converting the metallic and albedo into the specular within the shader.