Support for VP8 and VP9 video codec

[ming]

Add support for VP8 and VP9 video codec using ​libvpx.

During our testing, for the same bitrate, VP8 quality seems to be better than H264, but the encoding speed is noticeably slower. For VP9 however, as documented by several other sources as well [1][2], the encoding speed is much much slower, even though we have followed the recommended setting for "Real-time CBR Encoding and Streaming" from ​their doc here. Because of this, VP9 is currently by default disabled. To enable it, you can set PJMEDIA_HAS_VPX_CODEC_VP9 compile-time macro.

[1] ​https://blogs.gnome.org/rbultje/2014/02/22/the-worlds-fastest-vp9-decoder-ffvp9/
[2] ​https://www.streamingmedia.com/Articles/ReadArticle.aspx?ArticleID=111334

Steps to use VP8/VP9

1. Download ​libvpx.
2. Build and install. General build steps can be found in libvpx README file.
   • For Android, these steps can be used:

     export NDK=/path/to/Android-NDK
     export HOST_TAG=linux-x86_64 # adjust to your building host
     
     export TOOLCHAIN=$NDK/toolchains/llvm/prebuilt/$HOST_TAG
     export AR=$TOOLCHAIN/bin/aarch64-linux-android-ar
     export AS=$TOOLCHAIN/bin/aarch64-linux-android-as
     export CC=$TOOLCHAIN/bin/aarch64-linux-android21-clang
     export CXX=$TOOLCHAIN/bin/aarch64-linux-android21-clang++
     export LD=$TOOLCHAIN/bin/aarch64-linux-android-ld
     export RANLIB=$TOOLCHAIN/bin/aarch64-linux-android-ranlib
     export STRIP=$TOOLCHAIN/bin/aarch64-linux-android-strip
     
     ./configure --target=arm64-android-gcc --enable-pic --disable-examples \
                 --disable-tools --disable-docs --enable-realtime-only \
                 --disable-install-bins --disable-unit-tests --disable-webm-io \
                 --disable-libyuv \
                 --prefix=/path/to/target/installation
     

3. Libvpx will then be autodetected by PJSIP configure script (alternatively, you can specify --with-vpx option).

[ming]

In 6110:

Fixed #2253: Support for VP8 and VP9 video codec

[riza]

In 6147:

Re #2253:

• Add vpx.h/vpx.c to Visual Studio project files
• Fix compile error/warning from Visual Studio

[ming]

In 6149:

Re #2253
Fix vpx fmtp apply:

• Only adjust the frame rate if it exceeds max-fr
• Maintain aspect ratio when adjusting resolution

[ming]

In 6150:

Re #2253: Apply local fmtp setting

[ming]

In 6151:

Re #2253: Changed keyframe max distance to a max of 60s

[ming]

In 6153:

Re #2253: Check for insufficient buffer if remote is sending higher res than we can handle.