Cycles: memory usage report

This commit adds memory usage information while rendering.

It reports memory used by device, meaning:

- For CPU it'll report real memory consumption
- For GPU rendering it'll report GPU memory consumption, but it'll
  also mean the same memory is used from host side.

This information displays information about memory requested by Cycles,
not memory really allocated on a device. Real memory usage might be
higher because of memory fragmentation or optimistic memory allocator.

There's really nothing we can do against this.

Also in contrast with blender internal's render cycles memory usage
does not include memory used by scene, only memory needed by cycles
itself will be displayed. So don't freak out if memory usage reported
by cycles would be much lower than blender internal's.

This commit also adds RenderEngine.update_memory_stats callback which
is used to tell memory consumption from external engine to blender.
This information is used to generate information line after rendering
is finished.

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

@@ -115,6 +115,7 @@ void RE_engine_end_result(RenderEngine *engine, struct RenderResult *result, int
 int RE_engine_test_break(RenderEngine *engine);
 void RE_engine_update_stats(RenderEngine *engine, const char *stats, const char *info);
 void RE_engine_update_progress(RenderEngine *engine, float progress);
+void RE_engine_update_memory_stats(RenderEngine *engine, float mem_used, float mem_peak);
 void RE_engine_report(RenderEngine *engine, int type, const char *msg);
 
 int RE_engine_render(struct Render *re, int do_all);