Replaces hardcoded WEBP_FPS=15/GIF_FPS=15 with source_fps() that reads mf-fps, video-params/fps, or estimated-vf-fps, flooring at 30.
Replaces hardcoded WEBP_FPS=15/GIF_FPS=15 with source_fps() that reads mf-fps, video-params/fps, or estimated-vf-fps, flooring at 30.