Added support for the WEBM/VP9 video codec

WEBM is the codec name, and VP9 is the encoder (the older encoder "VP8"
is less efficient than VP9).

WEBM/VP9 and h.264 both have options to control the file size versus
compression time (e.g. fast but big, or slow and small, for the same
output quality). Since WEBM/VP9 only has three choices, I've chosen to
map those to 3 of the 9 possible choices of h.264:

- BEST → SLOWER
- GOOD → MEDIUM
- REALTIME → SUPERFAST

The VERYSLOW and ULTRAFAST options give very little extra benefit.

Reviewed by: @Severin

release/scripts/startup/bl_ui/properties_render.py

@@ -464,7 +464,7 @@ class RENDER_PT_encoding(RenderButtonsPanel, Panel):
             layout.prop(ffmpeg, "use_lossless_output")
 
         # Output quality
-        use_crf = needs_codec and ffmpeg.codec in {'H264', 'MPEG4'}
+        use_crf = needs_codec and ffmpeg.codec in {'H264', 'MPEG4', 'WEBM'}
         if use_crf:
             layout.prop(ffmpeg, "constant_rate_factor")