libhb: Unify VCN quality tuning for all encoders.

For the H.264 encoder (and now the H.265 encoder), sets P=I+2 and B=P+2.
Previously these were set to I=P and B=P+2. Adopts the offset taper as
added with the AV1 encoder, so P and B offset from I are reduced as I
approaches 0, ensuring max quality is achievable for all frame types.

Removes the extensive H.265 encoder tuning added in HandBrake 1.4.0
to work around major limitations in drivers and hardware available at
the time, e.g. RX 580. While still useful in some scenarios, it is no
longer strictly necessary to achieve reasonable quality.