vaapih264dec: Add a property to assume constrained-baseline
When baseline-as-constrained is set, the decoder will expose support for baseline decoding and assume that the baseline content is constrained-baseline. This can be handy to decode streams in hardware that would otherwise not be possible to decode. A lot of baseline content is in fact constrained.
@vjaquez This property enables a hack, I'm not sure if it make sense to mainline this, but I had to write it anyway so I thought maybe I should share it.