Before this change, the value of the `dwLength` in the stream header was
interpreted as the number of chunks in the file. Seeking and timestamp
calculation use the media duration and total chunk count. However, in some
files the `dwLength` field appears not to store the number of chunks. For
example, there are CBR MP3 and AC3 files where this field seems to store the
total number of bytes of compressed media instead. That caused seeking and
timestamp calculation to give much smaller values than expected (because the
`dwLength` is very large), which broke seeking.
Work around this using the `idx1` index header if present. We only support
audio formats where every audio sample is a sync sample in AVI, and all chunks
should therefore be listed in this index. Based on testing on many sample AVI
files this gives a reliable total chunk count and fixes seeking.
The test media file is a transcoded version of Big Buck Bunny but manually
edited to overwrite the length, rate and sample size header files to simulate
the error case.
PiperOrigin-RevId: 705103651
da05a1a66b