libs: encoder: h265: Deprecate the low-delay-b property.
libs: encoder: h265: Deprecate the low-delay-b property.

In HEVC, P and B definitions are different from AVC: P frames have
just one reference list and so 1 MV, while B frames have two reference
lists and so 2 MVs. No matter B or P, ist reference lists can contain
forward/backward reference. So P and B can both have bi-directions
dependency, the difference is just their reference list
number (i.e. MV number). This is different from the AVC.

The *low delay b mode* refers to a special HEVC mode, in which the
stream just contain I and B frames, without P frames, and all B frames
only have forward direction dependencies (i.e. all inter frames have 2
reference lists but no backward reference in both lists).  This is
similar to AVC I/P mode, but changing the P to the forward dependent
B.

The `low-delay-b` property is now just used to simply convert all P
frames to B frames when driver does not support P frames (so both
reference lists have the same references frames). This is a little
different from the meaning of low delay b mode (the two ref lists may
have the different reference frames). And the driver now can report
whether it supports P frames correctly, so there is no need to use
this property and deprecate it.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-vaapi/-/merge_requests/284>

GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi