Use libvpx-vp9 for main decode too

An extra pass was added to the ffmpeg_frame_generation code to properly
detect and handle alpha when loading vp9 videos. This early commit
allowed for correct detection, but then failed to apply the decoder to
the main pass, so no alpha was output. This has been fixed.

videohelpersuite/load_video_nodes.py

@@ -170,7 +170,8 @@ def cv_frame_generator(video, force_rate, frame_load_cap, skip_first_frames,
 def ffmpeg_frame_generator(video, force_rate, frame_load_cap, start_time,
                            custom_width, custom_height, downscale_ratio=8,
                            meta_batch=None, unique_id=None):
-    args_dummy = [ffmpeg_path, "-i", video, '-c', 'copy', '-frames:v', '1', "-f", "null", "-"]
+    args_input = ["-i", video]
+    args_dummy = [ffmpeg_path] + args_input +['-c', 'copy', '-frames:v', '1', "-f", "null", "-"]
     size_base = None
     fps_base = None
     try:
@@ -181,7 +182,8 @@ def ffmpeg_frame_generator(video, force_rate, frame_load_cap, start_time,
                 + e.stderr.decode(*ENCODE_ARGS))
     lines = dummy_res.stderr.decode(*ENCODE_ARGS)
     if "Video: vp9 " in lines:
-        args_dummy = args_dummy[:1] + ["-c:v", "libvpx-vp9"] + args_dummy[1:]
+        args_input = ["-c:v", "libvpx-vp9"] + args_input
+        args_dummy = [ffmpeg_path] + args_input +['-c', 'copy', '-frames:v', '1', "-f", "null", "-"]
         try:
             dummy_res = subprocess.run(args_dummy, stdout=subprocess.DEVNULL,
                                      stderr=subprocess.PIPE, check=True)
@@ -214,15 +216,13 @@ def ffmpeg_frame_generator(video, force_rate, frame_load_cap, start_time,
     if start_time > 0:
         if start_time > 4:
             post_seek = ['-ss', '4']
-            pre_seek = ['-ss', str(start_time - 4)]
+            args_input = ['-ss', str(start_time - 4)] + args_input
         else:
             post_seek = ['-ss', str(start_time)]
-            pre_seek = []
     else:
-        pre_seek = []
         post_seek = []
-    args_all_frames = [ffmpeg_path, "-v", "error", "-an"] + pre_seek + \
-            ["-i", video, "-pix_fmt", "rgba64le"] + post_seek
+    args_all_frames = [ffmpeg_path, "-v", "error", "-an"] + \
+            args_input + ["-pix_fmt", "rgba64le"] + post_seek
 
     vfilters = []
     if force_rate != 0: