Cycles: Implement denoising option for reducing noise in the rendered image

This commit contains the first part of the new Cycles denoising option,
which filters the resulting image using information gathered during rendering
to get rid of noise while preserving visual features as well as possible.

To use the option, enable it in the render layer options. The default settings
fit a wide range of scenes, but the user can tweak individual settings to
control the tradeoff between a noise-free image, image details, and calculation
time.

Note that the denoiser may still change in the future and that some features
are not implemented yet. The most important missing feature is animation
denoising, which uses information from multiple frames at once to produce a
flicker-free and smoother result. These features will be added in the future.

Finally, thanks to all the people who supported this project:

- Google (through the GSoC) and Theory Studios for sponsoring the development
- The authors of the papers I used for implementing the denoiser (more details
  on them will be included in the technical docs)
- The other Cycles devs for feedback on the code, especially Sergey for
  mentoring the GSoC project and Brecht for the code review!
- And of course the users who helped with testing, reported bugs and things
  that could and/or should work better!

intern/cycles/kernel/kernel_path_state.h

@@ -35,6 +35,16 @@ ccl_device_inline void path_state_init(KernelGlobals *kg,
 	state->transmission_bounce = 0;
 	state->transparent_bounce = 0;
 
+#ifdef __DENOISING_FEATURES__
+	if(kernel_data.film.pass_denoising_data) {
+		state->flag |= PATH_RAY_STORE_SHADOW_INFO;
+		state->denoising_feature_weight = 1.0f;
+	}
+	else {
+		state->denoising_feature_weight = 0.0f;
+	}
+#endif  /* __DENOISING_FEATURES__ */
+
 	state->min_ray_pdf = FLT_MAX;
 	state->ray_pdf = 0.0f;
 #ifdef __LAMP_MIS__
@@ -128,6 +138,10 @@ ccl_device_inline void path_state_next(KernelGlobals *kg, ccl_addr_space PathSta
 
 	/* random number generator next bounce */
 	state->rng_offset += PRNG_BOUNCE_NUM;
+
+	if((state->denoising_feature_weight == 0.0f) && !(state->flag & PATH_RAY_SHADOW_CATCHER)) {
+		state->flag &= ~PATH_RAY_STORE_SHADOW_INFO;
+	}
 }
 
 ccl_device_inline uint path_state_ray_visibility(KernelGlobals *kg, PathState *state)