avcodec: allow frame threading with hardware acceleration
avcodec: allow frame threading with hardware acceleration

Only MPEG1/2 slice threading is currently known as broken.

This rectifies commit df252eac618091d156cf986f38a98c2bddbc8437.

jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,vlc-mirror/vlc,xkfz007/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.2,krichter722/vlc,shyamalschandra/vlc,vlc-mirror/vlc,krichter722/vlc,xkfz007/vlc,shyamalschandra/vlc,krichter722/vlc,vlc-mirror/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,vlc-mirror/vlc,shyamalschandra/vlc,krichter722/vlc,vlc-mirror/vlc,vlc-mirror/vlc,shyamalschandra/vlc,shyamalschandra/vlc,shyamalschandra/vlc,xkfz007/vlc,xkfz007/vlc,krichter722/vlc,xkfz007/vlc,xkfz007/vlc,jomanmuk/vlc-2.2,krichter722/vlc,jomanmuk/vlc-2.2,xkfz007/vlc,krichter722/vlc