On2 VP7 decoder

Further performance improvements and security fixes by
Vittorio Giovara, Luca Barbato and Diego Biurrun.

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
Signed-off-by: Diego Biurrun <diego@biurrun.de>
fate: add a VP7 test
arm: asm decode_block_coeffs_internal is vp8 specific

Unbreaks compilation on arm due to conflicting types for
'ff_decode_block_coeffs_armv6'.