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)

src/engines/core/base_engine.py

@@ -1,5 +1,6 @@
 import logging
 import os
+import platform
 import subprocess
 
 logger = logging.getLogger()
@@ -61,8 +62,9 @@ class BaseRenderEngine(object):
         path = self.renderer_path()
         if not path:
             raise FileNotFoundError("renderer path not found")
+        creationflags = subprocess.CREATE_NO_WINDOW if platform.system() == 'Windows' else 0
         help_doc = subprocess.check_output([path, '-h'], stderr=subprocess.STDOUT,
-                                           timeout=SUBPROCESS_TIMEOUT).decode('utf-8')
+                                           timeout=SUBPROCESS_TIMEOUT, creationflags=creationflags).decode('utf-8')
         return help_doc
 
     def get_project_info(self, project_path, timeout=10):