Blender Git Loki

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