use picture-level new seq begin detection for output functions
Captizalize OpenH264 correctly in the version header
Add a public function for getting the codec version

If the codec library is loaded dynamically, it may be necessary that
the loaded library actually matches the version that was used when
it was built, to avoid issues due to ABI differences. Therefore add
a public function to allow getting the version of the currently
loaded library.
Merge pull request #1651 from mstorsjo/improve-version-header

Improve version header
Merge pull request #1652 from huili2/newseq_bugfix

use picture-level new seq begin detection for output functions
remove macro definition to the header file
Merge pull request #1653 from ruil2/framerate

remove macro definition to the header file
maintain codec demo
Merge pull request #1654 from syureyi/newest

maintain codec demo
add release note for preparation of v1.3.0
Merge pull request #1655 from sijchen/docx

add release note for preparation of v1.3.0