libvpx: bump version to 1.4.0

Use tar.bz2 instead of git.
Also added the hash file.
Changes: https://chromium.googlesource.com/webm/libvpx/+/0080aca235e2d677e49ff46c3e8e77d9dd15858c/CHANGELOG



[Thomas: remove unnecessary LIBVPX_VERSION_MAJOR, as suggested by
Arnout.]

Signed-off-by: Sagaert Johan <sagaert.johan@proximus.be>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>