Correctly flush at the end of OBUs

This fixes failures when an OBU has more than a byte's worth of trailing zeros.

As part of this work, it also rejigs the dav1d_flush_get_bits function slightly. This worked before, but it wasn't very obvious why (it worked because bits_left was never more than 7). This patch renames it to dav1d_bytealign_get_bits, which makes it clearer what it does and adds a comment explaining why it works properly.

The new dav1d_bytealign_get_bits is also now void (rather than returning the next byte to read). The patch defines dav1d_get_bits_pos, which returns the current bit position. This feels a little easier to reason about.

We also add a new check to make sure that we haven't fallen off the end of the OBU. This can happen when a byte buffer contains more than one OBU: the GetBits might not have got to EOF, but we might now be half-way through the next OBU.

This fixes issue #167 (closed)