Fix refraction in crown scene.

Via jonasdeyson@, https://github.com/mmp/pbrt-v3/issues/129.

crown/crown.pbrt

@@ -1,3 +1,4 @@
+Scale -1 1 1
 
 Film "image" "integer xresolution" 1000 "integer yresolution" 1400
     "string filename" "crown.exr"
@@ -9,12 +10,11 @@ Camera "perspective" "float fov" 47
   "float focaldistance" 17 "float lensradius" .3
 
 Sampler "sobol" "integer pixelsamples" 16
-#Sampler "sobol" "integer pixelsamples" 64
+Sampler "sobol" "integer pixelsamples" 64
 Sampler "sobol" "integer pixelsamples" 4096
 Integrator "path" "integer maxdepth" 20
 
 WorldBegin
-Scale -1 1 1
 
 # Name "back_light"
 AttributeBegin