Refactor: Cycles: weight reflection/refraction lobe selection pdf by tint

Now that there are different Fresnel types and the reflectance can be tinted,
it is better to sample based on the tinted instead of the real Fresnel. Also
avoid computing Fresnel multiple times.

@@ -511,3 +517,2 @@
-      (is_transmission && m_reflection) || (!is_transmission && m_refraction))
+      (is_transmission && !has_transmission) || (!is_transmission && !has_reflection))
   {
-    *pdf = 0.0f;

@@ -219,1 +210,3 @@
-{
+ * If generalized Schlick is used, the reflectance is an interpolation of the F0 color and the F90
+ * color.
+ * For dielectric and conductor, use Fresnel equations.

@@ -220,0 +213,4 @@
+ * Otherwise returns white. */
+ccl_device_forceinline void microfacet_fresnel(ccl_private const MicrofacetBsdf *bsdf,
+                                               const float cos_theta_i,
+                                               ccl_private float *r_cos_theta_r,

@@ -254,3 +254,1 @@
-          return refraction ? zero_spectrum() : fresnel->reflection_tint;
-        }
-        cosI = safe_sqrtf(1.0f - sinT2);
+      const float cosT_sq = 1.0f - (1.0f - sqr(cos_theta_i)) / sqr(bsdf->ior);

@@ -257,0 +258,4 @@
+        *r_transmission = zero_spectrum();
+        return;
+      }
+      const float cosT = safe_sqrtf(cosT_sq);

@@ -258,2 +266,3 @@
       /* TODO(lukas): Is a special case for exponent==5 worth it? */
-      s = powf(1.0f - cosI, fresnel->exponent);
+      /* When going from a higher to a lower IOR, we must use the transmitted angle. */
+      s = powf(1.0f - ((bsdf->ior < 1.0f) ? *r_cos_theta_r : cos_theta_i), fresnel->exponent);