Orpington Duck Release (v1.8.1)

[Johann Koenig]

The Orpington Duck release has been finalized.

Since the initial release candidate changes have been merged to:
vp9-rtc: Fix color artifacts for speed >= 8
vp9_cx_iface,encoder_encode: fix -Wclobbered for pts

The release is available from the git repository[0] on the orpington
branch and the automatic v1.8.1 tarball[1] which should have a
deterministic hash[2].

This release collects incremental improvements to many aspects of the library.

- Upgrading:
VP8E_SET_CPUUSED now accepts values up to 9 for vp9.
VPX_CTRL_VP9E_SET_MAX_INTER_BITRATE_PCT had a spelling fix (was VP8E).
The --sdk-path option has been removed. If you were using it to build for
Android please read build/make/Android.mk for alternatives.
All PPC optimizations have been disabled:
https://bugs.chromium.org/p/webm/issues/detail?id=1522.

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

- Bug fixes:
vp9-rtc: Fix color artifacts for speed >= 8.

[0] https://chromium.googlesource.com/webm/libvpx/+/v1.8.1
[1] https://chromium.googlesource.com/webm/libvpx/+archive/v1.8.1.tar.gz
md5: ba8dd72c1e066a3b950731b5f3b44dab
sha1: cc14ba22f60df7ef6acce51ed0ffb0c280b28b1e
[2] https://github.com/google/gitiles/issues/84#issuecomment-503897403