Add option to write constraint_set4_flag/constraint_set5_flag in the SPS

x264 briefly did this, starting with commit 92d36908, but that was reverted in f9af2a0f because it caused issues with some hardware decoders that assume those bits are always zero.

This re-adds writing those bits, but gated under an option, which is off by default, since sometimes we may actually want to write those bits properly.