Cycles: Add support for debug passes

Currently only summed number of traversal steps and intersections used by the
camera ray intersection pass is implemented, but in the future we will support
more debug passes which would help checking what things makes the scene slow.
Example of such extra passes could be number of bounces, time spent on the
shader tree evaluation and so.

Implementation from the Cycles side is pretty much straightforward, could only
mention here that it's a build-time option disabled by default.

From the blender side it's implemented as a PASS_DEBUG with several subtypes
possible. This way we don't need to create an extra DNA pass type for each of
the debug passes, saving us a bits.

Reviewers: campbellbarton

Reviewed By: campbellbarton

Differential Revision: https://developer.blender.org/D813

source/blender/render/extern/include/RE_pipeline.h

@@ -72,8 +72,13 @@ typedef struct RenderPass {
 	char chan_id[8];	/* amount defined in openexr_multi.h */
 	float *rect;
 	int rectx, recty;
+	int debug_type;
 } RenderPass;
 
+enum {
+	RENDER_PASS_DEBUG_BVH_TRAVERSAL_STEPS = 0,
+};
+
 /* a renderlayer is a full image, but with all passes and samples */
 /* size of the rects is defined in RenderResult */
 /* after render, the Combined pass is in rectf, for renderlayers read from files it is a real pass */