[Bug 788918] vaapi: supports hierarchical prediction encoding

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=788918

--- Comment #8 from Hyunjun Ko <zzoon at igalia.com> ---
Created attachment 361494
  --> https://bugzilla.gnome.org/attachment.cgi?id=361494&action=edit
libs: encoder: h264: Add Hierarchical-B encode

Frames are encoded as differnt layers. Frame in a particular
layer will use pictures in lower or same layer as references.
Which means decoder can drop the frames in upper layer ,but still
decode lower layer frames.

B-frames, except the one in top most layer are reference frames, all 
th base layer frames are I or P.

eg: with 3 temporal layers

    T3:             B1            B3              B5              B7

    T2:                   B2                              B6

    T1:   I0                                P4                        P8

    T1, T2, T3: Temporal Layers
    P1...Pn:   P-Frames:
    B1...Bn:   B-frames:
    T1: I0->P4 , P4->P8 etc..
    T2: I0--> B2 <-- P4
    T3: I0--> B1 <-- B2, B2 --> B3 <-- P4

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