Allow ffmpeg multi-core decoding when hardware accelerate switches are all off

ffmpeg-mt should be used when no hardware accelerate available or usable. but current code only allow it for hi10p.

it's possible to apply ffmpeg-mt when no hardware accelerate switch is on, that means either the system does not support hardware accelerate, or user switched off the hw accel manually. on both situations, user can benefit from the multi-core decoding.