Metalness in DBuffer decals

[MothDoctor]

Mhm… this comes from the latest Dev-Rendering merge.
https://github.com/EpicGames/UnrealEngine/commit/e3ec1a0d945833a644c62bcda321b51b4f168df3

Change 4183727 by Yuriy.ODonnell Implemented auto-conversion from deferred to DBuffer decals in forward shading mode (when GBuffer is not available). Added support for specular and metallic channels for DBuffer decals, based on work by Chris Bunner. This requires DBufferC to be expanded from 2 to 4 channels, leading to slight increase in DBuffer bandwidth and memory requirements. Appearance of DBuffer decals is affected by this change, as specular and metallic channel values previously hard-coded in DBufferDecalShared.ush. Decals were forced to be non-metallic and have specular of 4% (0.5 numeric value). Now the authored decal material values will be used, which matches GBuffer decals. Added support for DBuffer decals with emissive component. Most decal types can now be automatically converted, with the exception of stain decals. Those are currently approximated as regular translucent decals.