MP3: detect headerless VBR after parsing finishes

[bernardwiesner]

Headerless MP3 with late bitrate change is incorrectly reported as CBR

MediaInfoLib reports a headerless MP3 as CBR when its first frames use one bitrate but later frames use different bitrates.

This occurs with both the default ParseSpeed and ParseSpeed=1.0.

Reproduction

Use an MP3 without Xing, Info, or VBRI metadata. The file should begin with CBR frames and switch to VBR frames later.

Example fixture structure:

• First 30 seconds: CBR MP3 at 128 kbps
• Final 30 seconds: VBR MP3
• No Xing, Info, or VBRI header

kalimba_60s_cbr_then_vbr_no_xing.mp3

Run:

mediainfo --Output=JSON kalimba_60s_cbr_then_vbr_no_xing.mp3

The audio stream reports:
"BitRate_Mode": "CBR"

Expected result:
"BitRate_Mode": "VBR"

Problem

File_Mpega::Streams_Fill() detects headerless VBR files by checking whether more than one bitrate has been observed:

if (BitRate_Count.size()>1)
    BitRate_Mode=__T("VBR");

This check runs after the initial frames are parsed. If those frames all use the same bitrate, the stream is initially classified as CBR.

Later frames continue to update BitRate_Count, including frames inspected after the default tail seek. However, the bitrate mode is not re-evaluated during finalization.

As a result, a headerless MP3 that begins with CBR frames and changes bitrate later can incorrectly remain classified as CBR.