Rudimentary CI for Ubuntu and macOS

[speedy-beaver]

Two Github Actions to build main branch on Ubuntu and macOS on push to main Adding badges to the Readme.md to indicate CI status for main branch

[hzeller]

Thanks, merged!

If you're up for it, I have some suggestions for further improvements to help detect corner-cases:

• Compile with -Werror so that warnings will result in a failed build.
• Compile with both, CXX=g++ and CXX=clang++ to see if there are any differences of things the one compiler detects and the other does not. With what is availbable on the platform of course (not sure if brew also does gcc or just clang).
• Compile with both, -DWITH_VIDEO_DECODING=On and -DWITH_VIDEO_DECODING=Off, so that it is possible to see if all #ifdefs are properly in place.

AFAIK, there is a feature called matrix that allows to set you a set of independent variables to cycle through, and it builds the crross product of all of these. So with the following setting, we'd get 4 builds g++/On, g++/Off, clang++/On, clang++/Off

  compiler: [ g++ clang++ ]
  video_decoding: [ On Off ]