Cycles: oneAPI: Fix prevented execution with sycl runtime > 20230323

NanoVDB headers have unused code using "double" type, which is not supported on Arc GPUs.
Recent DPC++ changes enforced runtime verifications:
7663dc201d
which prevents execution when such type has been present even if unused.
This is a solution to avoid double to be compiled at all, similar as how it is done for Metal.

intern/cycles/kernel/device/oneapi/context_begin.h

@@ -2,8 +2,40 @@
  * Copyright 2021-2022 Intel Corporation */
 
 #ifdef WITH_NANOVDB
+/* Data type to replace `double` used in the NanoVDB headers. Cycles don't need doubles, and is
+ * safer and more portable to never use double datatype on GPU.
+ * Use a special structure, so that the following is true:
+ * - No unnoticed implicit cast or mathematical operations used on scalar 64bit type
+ *   (which rules out trick like using `uint64_t` as a drop-in replacement for double).
+ * - Padding rules are matching exactly `double`
+ *   (which rules out array of `uint8_t`). */
+typedef struct ccl_vdb_double_t {
+  union ccl_vdb_helper_t {
+    double d;
+    uint64_t i;
+  };
+
+  uint64_t i;
+  ccl_vdb_double_t(double value)
+  {
+    ccl_vdb_helper_t helper;
+    helper.d = value;
+    i = helper.i;
+  }
+  /* We intentionally allow conversion to float in order to workaround compilation errors
+   * for defined math functions that take doubles. */
+  operator float() const
+  {
+    ccl_vdb_helper_t helper;
+    helper.i = i;
+    return (float)helper.d;
+  }
+} ccl_vdb_double_t;
+
+#  define double ccl_vdb_double_t
 #  include <nanovdb/NanoVDB.h>
 #  include <nanovdb/util/SampleFromVoxels.h>
+#  undef double
 #endif
 
 /* clang-format off */