Orpington Duck Release Candidate

[Johann Koenig]

The Orpington Duck is a hybrid composed of ducks from previous libvpx
releases: Aylesbury (v0.9.5), Cayuga (v0.9.7), and Indian Runner
(v1.4.0). We will be finalizing a v1.8.1 release soon. If you have any
issues with the current branch please let us know.

It is ABI compatible with v1.8.0 with some minor updates:
VP8E_SET_CPUUSED now accepts values up to 9 for vp9.
VP8E_SET_ENABLEAUTOALTREF documentation has been clarified regarding
ranges for vp8 and vp9.
VPX_CTRL_VP9E_SET_MAX_INTER_BITRATE_PCT has a spelling fix (was VP8E).

Bug fixes:
ppc vsx optimizations have been disabled.

Improvements:
Various changes to improve encoder rate control, quality and speed
for practically every use case.

Build fixes:
The --sdk-path option has been removed. If your project used it to
build for Android, please switch to --enable-external-build and use
ndk-build.

You can get the code with:
$ git clone https://chromium.googlesource.com/webm/libvpx
$ cd libvpx
$ git checkout orpington