Fix decoder to handle display size correctly

The decoder ignored the display width & height
specified in the frame header.

This patch adds a control, VP9D_GET_DISPLAY_SIZE, to
allow the application to obtain the display width and
height from the frame header.

vpxdec has been modified to scale the output frame to
this size.

Should the request for the display size fail vpxdec will
use the native width and height of the raw decoded
frame instead.

Change-Id: I25db04407426dac730263720c75a7dd6400af68a