AV1-ISOBMFF spec is updated recently to use start alignment instead of
end alignment for cbcs:
for the protected scheme cbcs:
- BytesOfProtectedData SHALL start on the first byte and end on the
last byte of the decode_tile structure (including any trailing bits).
- A subsample SHALL be created for each tile, even if its size is less
than 16 bytes.
- ...
cenc protection scheme is not affected.
Closes#698.
Change-Id: Ic83a478fb2602d830c30daf3206a1c2d2c238a08
ce932f68a2