Fix OpenImageIO thread pool not following -t command line option

We do this for the OpenEXR thread pool as well. Needed until OIIO fully
switches over to TBB.

Pull Request: https://projects.blender.org/blender/blender/pulls/110501

source/blender/imbuf/intern/filetype.cc

@@ -239,6 +239,8 @@ void imb_filetypes_init()
 {
   const ImFileType *type;
 
+  OIIO_init();
+
   for (type = IMB_FILE_TYPES; type < IMB_FILE_TYPES_LAST; type++) {
     if (type->init) {
       type->init();

source/blender/imbuf/intern/oiio/openimageio_api.cpp

@@ -7,12 +7,24 @@
  */
 
 #include "openimageio_api.h"
+
 #include <OpenImageIO/imageio.h>
 
+#include "BLI_threads.h"
+
 OIIO_NAMESPACE_USING
 
 extern "C" {
 
+void OIIO_init()
+{
+  /* Make OIIO thread pool follow Blender number of threads override. */
+  const int threads_override = BLI_system_num_threads_override_get();
+  if (threads_override) {
+    OIIO::attribute("threads", threads_override);
+  }
+}
+
 int OIIO_getVersionHex()
 {
   return openimageio_version();

source/blender/imbuf/intern/oiio/openimageio_api.h

@@ -12,6 +12,14 @@
 extern "C" {
 #endif
 
+/*
+ * Initialize OpenImageIO on startup.
+ */
+void OIIO_init(void);
+
+/*
+ * Get OpenImageIO version.
+ */
 int OIIO_getVersionHex(void);
 
 #ifdef __cplusplus