Move embree symbols into mitsuba namespace to prevent symbol conflicts (#1663)

This commit fixes symbol clashes that were encountered when using Mitsuba from within Blender's Python environment (as in the Blender plugin)

Author: dvicini

ext/CMakeLists.txt

@@ -93,6 +93,7 @@ if (MI_ENABLE_EMBREE)
   set(EMBREE_MAX_INSTANCE_LEVEL_COUNT  1 CACHE STRING " " FORCE)
   set(EMBREE_TASKING_SYSTEM            "INTERNAL" CACHE STRING " " FORCE)
   set(EMBREE_ISA_SSE2                  OFF CACHE BOOL " " FORCE)
+  set(EMBREE_API_NAMESPACE             "mitsuba" CACHE STRING " " FORCE)
 
   if ((CMAKE_SYSTEM_PROCESSOR STREQUAL "arm64") OR (CMAKE_SYSTEM_PROCESSOR STREQUAL "aarch64"))
     set(EMBREE_ISA_SSE42            OFF CACHE BOOL " " FORCE)

ext/embree

@@ -319,16 +319,20 @@ NAMESPACE_END(filesystem)
 
 namespace fs = filesystem;
 
-NAMESPACE_END(mitsuba)
-
-extern "C" {
 #if defined(MI_ENABLE_EMBREE)
-    // Forward declarations for Embree
+extern "C" {
+    // Forward declarations for Embree (we import Embree into mitsuba's namespace
+    // to avoid name clashes with other libraries).
     typedef struct RTCDeviceTy* RTCDevice;
     typedef struct RTCSceneTy* RTCScene;
     typedef struct RTCGeometryTy* RTCGeometry;
+}
 #endif
 
+NAMESPACE_END(mitsuba)
+
+extern "C" {
+
 // =============================================================
 //! @{ \name Helper macros
 // =============================================================