rather than symlink the webm/vpx terms, use the same header as libvpx to
reference in-tree files
based on the discussion in:
https://codereview.chromium.org/12771026/
Change-Id: Ia3067ecddefaa7ee01550136e00f7b3f086d4af4
unused currently, but the intent is to allow each format to populate
exif/xmp/icc with cwebp then transferring it to the webp file.
Change-Id: I0514f62de52fa7f89c595ee7ef2ad7dced910a41