Fix Cycles device kernels containing debug assertation code

NanoVDB includes "assert.h" and makes use of "assert" in several places and since the compile
pipeline for CUDA/OptiX kernels does not define "NDEBUG" for release builds, those debug
checks were always added. This is not intended, so this patch disables "assert" for CUDA/OptiX
by defining "NDEBUG" before including NanoVDB headers.
This also fixes a warning about unknown pragmas in NanoVDB thrown by the CUDA compiler.

intern/cycles/kernel/kernels/cpu/kernel_cpu_image.h

@@ -18,6 +18,7 @@
 #define __KERNEL_CPU_IMAGE_H__
 
 #ifdef WITH_NANOVDB
+#  define NANOVDB_USE_INTRINSICS
 #  include <nanovdb/NanoVDB.h>
 #  include <nanovdb/util/SampleFromVoxels.h>
 #endif

intern/cycles/kernel/kernels/cuda/kernel_cuda_image.h

@@ -15,6 +15,8 @@
  */
 
 #ifdef WITH_NANOVDB
+#  define NDEBUG /* Disable "assert" in device code */
+#  define NANOVDB_USE_INTRINSICS
 #  include "nanovdb/NanoVDB.h"
 #  include "nanovdb/util/SampleFromVoxels.h"
 #endif