libvpx

Fix src frame buffer copy and extend

For input source with size that is not multiple of 8, the size is
rounded to 8 and saved in width or height, the original source sizes
are saved in crop_width and crop_height. This commit corrects the
computation of bottom and right extension amounts to use the orignal
sizes, hence crop_width and crop_height.

In addition, this commit also adds the missed initialization for
uv_crop_width and uv_crop_height.

This addresses issue #834

Change-Id: I084543ca7645a4964b88f7cf8ff668f517d3a39b