fix: refractionTransmitted need linear space (#2548)

packages/core/src/shaderlib/pbr/btdf.glsl

@@ -20,6 +20,9 @@
 
         // Sample the opaque texture to get the transmitted light
         vec3 refractionTransmitted = texture2D(camera_OpaqueTexture, refractionCoords).rgb;
+        #ifndef ENGINE_IS_COLORSPACE_GAMMA
+            refractionTransmitted = gammaToLinear(vec4(refractionTransmitted, 1.0)).rgb;
+        #endif
         refractionTransmitted *= material.diffuseColor;
          
         // Use specularFGD as an approximation of the fresnel effect