FS-7503 FS-7514 FS-7513: fix some buffering bugs in vlc and set mod_conference to match the rate and channels when calling vlc files

it generate better compatible mp4 than vlc, so it compatible with quicktime and safari
except that the first few video frames messed up, maybe some pts issue, to be fixed

support float audio, and it can save to webm

Get filehandles working with video in some cases (if using vlc://):
    mod_conference for play and record video (will record the canvas in mix mode or floor holder in non-mix mode)
    regular playback app should be able to play vlc streams

Add no-minimize-encoding member flag so particilar memebers can opt out of that setting and still get their own encoded stream (for bw related needs)

TODO:

  mod_vlc is a mess.  Find a way to merge video_context and file_context.  They are very similar and they are intertwined and messy.
  Find out why vlc creates messed up mp4 files that don't play everywhere
  Get VLC so it can record aac, mp4x webm

FS-7503 FS-7514: First crack at recording mp4 <action application="capture_video" data="vlc://#transcode{vcodec=h264,acodec=mp3}:std{access=file,mux=mp4,dst=/var/www/sounds/file.mp4}"/>

FS-7500: expand libpng detection to try to find a good png >= 1.6.16, fall back to older verison in weezy, and add support for building without libpng

FS-7509: add code to pick camera and mic from the page and ability to call video endpoints with no camera