When using the deferred lighting rendering pipeline, PBR texture sets cannot be used in conjunction with block texture variations. In practice, this issue means that deferred lighting enabled resource packs cannot use block texture variations.

Steps to reproduce:

• In a deferred lighting-enabled texture pack, assign a PBR texture set to a block that has multiple texture variations (or apply the provided "Variations Enabled" example pack, where dirt has been modified to include a texture set and texture variation)
• Observe that the PBR materials assigned by the texture set will be ignored
• To confirm that texture variation is the cause, remove the code in terrain_texture.json that assigns the texture variants (or apply the provided "Variations Disabled" example pack, which is the same as the "Variations Enabled" pack, except for the texture variation code being removed)
• Observe that PBR textures work correctly now that texture variations have been disabled

Observed behavior: when a resource pack has block texture variations defined in terrain_texture.json, PBR texture sets for that block will be ignored for by the game when using the deferred rendering pipeline. 

Expected behavior: assigning a PBR texture set to a texture variant should apply the material properties as it would to any other texture. PBR texture sets are defined per texture, not per block, and thus there is no clear reason why they should be incompatible with texture variations.