Cycles: Fix wrong shading on GPU when background has NaN pixels and MIS enabled

Quite simple fix for now which only deals with this case. Maybe we want to do
some "clipping" on image load time so regular textures wouldn't give NaN as
well.

intern/cycles/render/light.cpp

@@ -486,10 +486,18 @@ static void background_cdf(int start,
                            float2 *cond_cdf)
 {
 	/* Conditional CDFs (rows, U direction). */
+	/* NOTE: It is possible to have some NaN pixels on background
+	 * which will ruin CDF causing wrong shading. We replace such
+	 * pixels with black.
+	 */
 	for(int i = start; i < end; i++) {
 		float sin_theta = sinf(M_PI_F * (i + 0.5f) / res);
 		float3 env_color = (*pixels)[i * res];
 		float ave_luminance = average(env_color);
+		/* TODO(sergey): Consider adding average_safe(). */
+		if(!isfinite(ave_luminance)) {
+			ave_luminance = 0.0f;
+		}
 
 		cond_cdf[i * cdf_count].x = ave_luminance * sin_theta;
 		cond_cdf[i * cdf_count].y = 0.0f;
@@ -497,6 +505,9 @@ static void background_cdf(int start,
 		for(int j = 1; j < res; j++) {
 			env_color = (*pixels)[i * res + j];
 			ave_luminance = average(env_color);
+			if(!isfinite(ave_luminance)) {
+				ave_luminance = 0.0f;
+			}
 
 			cond_cdf[i * cdf_count + j].x = ave_luminance * sin_theta;
 			cond_cdf[i * cdf_count + j].y = cond_cdf[i * cdf_count + j - 1].y + cond_cdf[i * cdf_count + j - 1].x / res;