Support for OpenH264 codec

[bennylp]

Support for the open source implementation of H.264 codec from ​http://www.openh264.org.

Implementation features:

• packetization modes: 1, 0
• whole frame for file encoding (untested)
• key frame request and indication
• obey remote's fmtp

Notes:

• currently transmission is hard coded to use only single NAL (packetization mode 0) regardless of remote's fmtp for interop reason. The same is also done in ffmpeg wrapper.

Below are the steps to use the codec:

For GNU targets:

1. Run configure with --with-openh264 option specifying the directory of OpenH264 source files. E.g.:

   ./configure --with-openh264=/home/user/src/openh264-trunk
   

2. Add this to your config_site.h:

   #define PJMEDIA_HAS_OPENH264_CODEC		1
   

For Visual Studio:

1. Add this to your config_site.h:

   #define PJMEDIA_HAS_OPENH264_CODEC		1