Adds enums and corresponding vpx level code.

Change-Id: Ia402d47490a4466988d7edc6b7e3e5163f20a381

Adds support for raw yuv inputs in 422/444 sampling for use
in profiles 1 and 3.
New options added to vpxenc are:
--i422 and --i444, which are to be used in conjunction with
--width, --height, and --fps for proper raw yuv handling.
A new option is added to vpxdec:
--rawvideo, which enforces raw yuv video output for the
bit-stream decoded irrespective of 420, 422 or 444 sampling.
The existing options --i420 and --yv12
are specialized for use only for 420 content.

Change-Id: I2e3028380709afa673bf2e2c25ad5e271a626055

Change-Id: I73865bbbf6ea74434f15b73783ae97b4d273be80

Change-Id: I137e5e6585356792913e1e84da6c0a439c5153a5

Change-Id: I5164df72aff84eca0ace56032c5373f04053c6a5

- Rename the struct to VpxEncoderConfig.
- The idea behind this is to enable checking the global settings against
  stream specific settings in source files other than vpxenc.c.

Change-Id: Ic736cbb714845b9466acb34671780d65b83ad1a8