Fix bug in extend_frame chroma extended too far

This fixes issue 667.

In the case where the frame was an odd number of pixels
wide or high, the border was being extended by one col
or row too far.

The calculation of color plane dimensions was modified
to use those already computed at the time the frame
buffer was allocated.

Also freed the temporary scaling buffer in vpxdec to
prevent a memory leak.

Change-Id: I195bc81d84c0fc5d8260c1232200d62399e4b51f