constraint_set1_flag should be set for constrained baseline and main
profiles, because the stream conforms to main profile.

constraint_set3_flag should be set for high profile when the stream
is intra-only.

constraint_set4_flag should always be set for main and high profiles
because interlaced encoding is not supported.

constraint_set5_flag should be set for main and high profiles when
B-frames are not used.

Also fix the setting of max_num_ref_frames - use the gop_size value
to check for intra-only rather than the constraint flag (which is not
necessarily set).