fix: ImageInput/ImageOutput did not set per-file threads correctly (#4750)

II and IO were documented as initializing their internal "how many
threads to use when reading/writing this particular file" to the global
"threads" OIIO attribute. But they did not, they always initialized to 0
("all the threads") even if the global attribute had been set
differently prior to creation of the reader or writer.

---------

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Author: lgritz

src/libOpenImageIO/imageinput.cpp

@@ -35,6 +35,7 @@ class ImageInput::Impl {
 public:
     Impl()
         : m_id(++input_next_id)
+        , m_threads(pvt::oiio_threads)
     {
     }
 

src/libOpenImageIO/imageoutput.cpp

@@ -38,6 +38,7 @@ class ImageOutput::Impl {
 public:
     Impl()
         : m_id(++output_next_id)
+        , m_threads(pvt::oiio_threads)
     {
     }