libs: encoder: h264/5: determine num_ref_idx_active_override_flag according to refere...

libs: encoder: h264/5: determine num_ref_idx_active_override_flag according to reference list

Follows the specification as below:

7.4.7.1 in Rec. ITU-T H.265 v4 (12/2016)
num_ref_idx_active_override_flag equal to 1 specifies that the syntax
element num_ref_idx_l0_active_minus1 is present for P and B slices and
that the syntax element num_ref_idx_l1_active_minus1 is present for B
slices.
num_ref_idx_active_override_flag equal to 0 specifies that the syntax
elements num_ref_idx_l0_active_minus1 and num_ref_idx_l1_active_minus1
are not present.

https://bugzilla.gnome.org/show_bug.cgi?id=783804