API: h264 encode: add new VA config attributes (max-slices, slice-structure).
API: h264 encode: add new VA config attributes (max-slices, slice-structure).

gbeauchesne/libva,pma1/libva,pma1/libva,chenxianqin/vaapi-libva,chenxianqin/vaapi-libva,01org/iotg-lin-gfx-libva,zzhhui/libva,01org/libva-cros,01org/libva-cros,chenxianqin/vaapi-libva,01org/iotg-lin-gfx-libva,pma1/libva,zzhhui/libva,zzhhui/libva,01org/libva-cros,01org/iotg-lin-gfx-libva,gbeauchesne/libva,gbeauchesne/libva