Function buildLoudnormFilter

Build a loudnorm filter string.

measured accepts the result of parseLoudnorm directly (EbuR128Result, camelCase). The ffmpeg-native snake_case spelling (input_i, input_lra, input_tp, input_thresh, target_offset) is also accepted, since that is the spelling ffmpeg prints in its analysis output. targetOffset is optional — when omitted the offset= option is left out entirely, because emitting offset=undefined makes ffmpeg abort with Unable to parse option value "undefined".