Make x264_encoder_reconfig more threadsafe

Do the reconfig when the next frame's encode begins.
Fixes some rare crashes with frame-threading and encoder_reconfig.
Add --filler option

Allows generation of hard-CBR streams without using NAL HRD.
Useful if you want to be able to reconfigure the bitrate (which you can't do
with NAL HRD on).
Add AVC-Intra 1080p50/60 Class 100 parameters

Also add some compatibility fixes.
Add L-SMASH support as preferable alternative for MP4-muxing
Remove --visualize option.

It probably wasn't used or maintained for last few years.