FS-7500: add a framebuffer to reuse memory and use it to offload frame writing from video muxing thread to a dedicated write thread

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