Partial fix for T37604: Deadlock when stopping rendered viewport (Blender Internal)

- Py_BEGIN_ALLOW_THREADS and Py_END_ALLOW_THREADS macros cannot be used here, because the Py_BEGIN_ALLOW_THREADS causes a crash when quitting Blender.
- The low level function PyEval_ReleaseLock() is used assuming the Python library was built with multi-threads support.

source/blender/python/intern/bpy_interface.c

@@ -408,6 +408,31 @@ void BPY_python_reset(bContext *C)
 	BPY_modules_load_user(C);
 }
 
+/* wrapper functions related to global interpreter lock. these functions
+ * are slightly different from the original Python API, don't throw
+ * SIGABRT even if the thread state is NULL. */
+
+/* analogue of PyEval_SaveThread() */
+BPy_ThreadStatePtr BPY_thread_save(void)
+{
+	PyThreadState *tstate = PyThreadState_Swap(NULL);
+	/* note: tstate can be NULL when quitting Blender */
+
+	if (tstate && PyEval_ThreadsInitialized()) {
+		PyEval_ReleaseLock();
+	}
+
+	return (BPy_ThreadStatePtr)tstate;
+}
+
+/* analogue of PyEval_RestoreThread() */
+void BPY_thread_restore(BPy_ThreadStatePtr tstate)
+{
+	if (tstate) {
+		PyEval_RestoreThread((PyThreadState *)tstate);
+	}
+}
+
 static void python_script_error_jump_text(struct Text *text)
 {
 	int lineno;