Allow two L1 refs when b-pyramid is enabled

[Steve Borho]

Consider this common case: You have 5 consecutive (display order frames) that are determined to be P1-B1-B2-B3-P2 by the lookahead. When b-pyramid is enabled, the middle B will be encoded first and used as a reference by the two following B frames (in encode order); P1-P2-B2ref-B1-B3

frame	L0	L1
P1
P2	P1
B2ref	P1	P2
B1	P1	P2 B2ref
B3	B2ref P1	P2

When B1 is encoded, both B2ref and P2 should be available as L1 references but we are only using one of them.

This will require some care where we are signaling max DPB sizes and other places, but should in the end be a small change. When this is fixed, the encode compression efficiency should improve when b-pyramid is enabled

[Steve Borho]

dpb: Allow two L1 refs when b-pyramid is enabled [CHANGES OUTPUTS]

Consider this common case: if we have 5 consecutive (display order frames) that are determined to be P1-B1-B2-B3-P2 by the lookahead. When b-pyramid is enabled, the middle B will be encoded first and used as a reference by the two following B frames (in encode order); P1-P2-B2ref-B1-B3

frame L0 L1 P1 P2 P1 B2ref P1 P2 B1 P1 P2 B2ref B3 B2ref P1 P2

When B1 is encoded, both B2ref and P2 should be available as L1 references, this will improve the encode compression efficiency when b-pyramid is enabled (closes #12)

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