Enable vpxenc to specify internal coded frame size

Added command line flags "resize-width" & "resize-height"
to allow the user to specify the frame size to encode at.

These two flags are ignored if the "resize-allowed" switch
is not set to 1.

All frames in the clip are then encoded at this size, which
must be smaller than the raw frame size.

Change-Id: I3d64bd9303d5c0bd678461a866a1ea621700d744