Manually specify the output format with Shaka-Packager

It normally auto-detects the format from the file extension. The supports formats are "MP4" and "WEBM". The input files to shaka-packager are currently always ".mp4", so this isn't particularly an issue.

However, I want to add this just as a pre-caution in case it isn't. This isn't an issue if the input file is another format, like WEBM, as this only controls the output format, the format devine wants, not the input and output format.

devine/core/drm/widevine.py

@@ -235,7 +235,7 @@ class Widevine:
 
         try:
             arguments = [
-                f"input={path},stream=0,output={output_path}",
+                f"input={path},stream=0,output={output_path},output_format=MP4",
                 "--enable_raw_key_decryption", "--keys",
                 ",".join([
                     *[