Improved slide show coding
Improved slide show coding

This patch improves coding of slide shows with fade or other
complex transitions.

Previously, fades and other complex transitions between static "slides"
were sometimes being incorrectly marked such that they were coded
as a single static slide rather than two slides with a transition.

As the initial key frame for the first slide is not necessarily a good
predictor of the second slide and ARFs were turned off, this led to a
poor visual and metrics outcome in some such cases.

This patch allows for long GF groups in static sections before and after
a complex transition (instead of just with  simple slide transitions) with
one or more normal ARF groups during the transition. It also enforces a
single "normal" length GF group after the transition before any extended
group is allowed. The reason for this is that the ARF that spans the
transition my not have a very high quality and hence may not act as a
good GF for the long static section that follows.

Change-Id: Ica1f979e27d8a0625f3cebf7b7cf6d69edccaba9

webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx