Prevent subprocesses from constantly opening windows on Windows (#109)

* Add subprocess.CREATE_NO_WINDOW to blender_engine.py

* Convert ffmpeg_engine.py to use CREATE_NO_WINDOW

* Cleanup Blender implementation

* Cleanup subprocesses in base_worker.py

* Cleanup subprocesses in base_engine.py

* Fix main.spec for Windows (optimize=2 broke it)

main.spec

@@ -26,7 +26,7 @@ a = Analysis(
     runtime_hooks=[],
     excludes=[],
     noarchive=False,
-    optimize=2,
+    optimize=0,
 )
 pyz = PYZ(a.pure)