Change the strategy for deciding the display size

There are three contributors to the definition of how the
display size is set:

(1) display width/height set in the container.
(2) display size (optional in the frame header)
(3) decoded frame size (from the frame header)

This patch modifies the way that vpxdec defines the display
size to give preference to these three criteria in the order
given above. If the container sets a non-zero size, it is
used, otherwise the display size specified in the first
decoded frame is used (if specified), with the raw
decoded frame size of the first frame used as a last resort.

The display size set in frames other than the first is
always ignored in this implementation.

Change-Id: I7e98d817d3f5894d559dd2aeb0a6cb1959b9092b