Merge pull request #393 from KhronosGroup/fix/shader_volume_without_transmission

fixed shader for materials with volume extension but without transmission

Author: UX3D-haertl

source/Renderer/shaders/pbr.frag

@@ -156,7 +156,7 @@ void main()
 #endif
 #endif
 
-#if (defined(MATERIAL_TRANSMISSION) || defined(MATERIAL_VOLUME)) && (defined(USE_PUNCTUAL) || defined(USE_IBL))
+#if defined(MATERIAL_TRANSMISSION) && (defined(USE_PUNCTUAL) || defined(USE_IBL))
     f_transmission += materialInfo.transmissionFactor * getIBLVolumeRefraction(
         n, v,
         materialInfo.perceptualRoughness,