FFmpeg: Added writing of WebM containers

This commit adds support for the WebM container. Previously we only
supported the WebM/VP9 video codec, but still required that it was
stored in a Matroska, MP4, or other compatible container format.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D5156

release/scripts/startup/bl_ui/properties_output.py

@@ -376,7 +376,7 @@ class RENDER_PT_encoding_video(RenderOutputButtonsPanel, Panel):
         layout = self.layout
         ffmpeg = context.scene.render.ffmpeg
 
-        needs_codec = ffmpeg.format in {'AVI', 'QUICKTIME', 'MKV', 'OGG', 'MPEG4'}
+        needs_codec = ffmpeg.format in {'AVI', 'QUICKTIME', 'MKV', 'OGG', 'MPEG4', 'WEBM'}
         if needs_codec:
             layout.prop(ffmpeg, "codec")