feat: Disable loading Python DLLs from PATH by default on Windows

Users can enable the legacy behavior by setting env var `OIIO_PYTHON_LOAD_DLLS_FROM_PATH=1`.

This commit also changes the environment variable name from `OIIO_LOAD_DLLS_FROM_PATH` to `OIIO_PYTHON_LOAD_DLLS_FROM_PATH` to match the convention used by OCIO.

src/python/__init__.py

@@ -7,8 +7,8 @@
 # This works around the python 3.8 change to stop loading DLLs from PATH on Windows.
 # We reproduce the old behaviour by manually tokenizing PATH, checking that the directories exist and are not ".",
 # then add them to the DLL load path.
-# This behviour can be disabled by setting the environment variable "OIIO_LOAD_DLLS_FROM_PATH" to "0"
-if sys.version_info >= (3, 8) and platform.system() == "Windows" and os.getenv("OIIO_LOAD_DLLS_FROM_PATH", "1") == "1":
+# This behviour can be enabled by setting the environment variable "OIIO_PYTHON_LOAD_DLLS_FROM_PATH" to "1"
+if sys.version_info >= (3, 8) and platform.system() == "Windows" and os.getenv("OIIO_PYTHON_LOAD_DLLS_FROM_PATH", "0") == "1":
     for path in os.getenv("PATH", "").split(os.pathsep):
         if os.path.exists(path) and path != ".":
             os.add_dll_directory(path)