libvpx

test/vpxenc.sh: Convert vpxenc() to a simple wrapper.

- Split vpxenc() into vpxenc() and vpxenc_pipe().
- Drop all but one positional param (the input file) in favor
  of passing args directly to vpxenc.
- Add an extra lossless test that explicitly sets min-q and
  max-q to 0.

Change-Id: I7d5f7b495f8b9447388c5f459bc9f6de2214caf2