FS-7499: move fir and pli into the normal rtcp code so it can be bundled with a report block per the rfc

FS-7509: vid screen share placeholder --args --enable-usermedia-screen-capturing --usermedia-screen-capturing

FS-7519 FS-7517: looks like decoders don't decode FU-A bit steams, this is a simple implementation that works with some FU-A bit streams, mod_av works better most cases

FS-7500: factor out the write_image callback and fix the vlc video endpoint to setup and match the audio params of the A leg when bridged