Added code for handling 4:2:2 and 4:4:4

These modes are supported by VP9 profile 1 (as opposed to the default
profile 0). I don't have any way for the user to specify these modes
yet, but will someday. Other programs don't seem to be able to handle
profile 1 yet.

vpx_image_alloc() is not making pixel worlds with these settings
properly as of now.