SoundFont 2: add option to keep length-mismatched stereo-linked samples as mono

[douglas-carmichael]

Problem

Some SoundFonts — in particular commercial E-mu banks such as the "Orbit" bank — carry unreliable stereo metadata: samples are flagged as left/right and linked (wSampleLink) to partners they are not actually the stereo halves of. When such a bank is converted, combineToStereo tries to weld these "pairs" back together.

compareSampleFormat already declines the merge when the two samples differ in pitch or sample rate, but a length mismatch only logged a warning and then merged them anyway — so two unrelated sounds ended up hard-panned into the left and right channels of a single stereo sample (the shorter one truncating the longer). Converting one such bank also produced a wall of warnings.

Change (updated per review)

Per @git-moss's feedback that some banks contain genuine stereo pairs whose left/right channels differ in length, the length-mismatch behaviour is now opt-in and off by default:

• New "Keep mismatched stereo samples as mono" option (Sf2KeepMismatchedStereoAsMono, off by default). When enabled, a length mismatch declines the stereo merge just like a pitch/sample-rate mismatch, so the two samples are kept as the separate mono samples they actually are. Off by default a length-mismatched pair still combines, so genuine different-length stereo pairs are unaffected.
• The pitch / sample-rate / length "left and right samples do not match" notices are now shown only when the existing Log unused SF2 generators option is enabled, so a normal conversion of a quirky bank stays clean while the detail is still available on request.

Since the stereo-combining happens while reading the SoundFont, the checkbox lives in the SF2 source options (next to "Log unused SF2 generators"); happy to move it if you'd prefer it elsewhere.

Rebased onto current main. The diff is limited to Sf2Detector.java, Sf2DetectorUI.java, Strings.properties, and the changelog / format docs.

Verification

Tested against a commercial E-mu "Orbit" SoundFont, converting every preset to WAV:

• Off (default): the 3 length-mismatched pairs are each combined into a single stereo file (unchanged from before this PR).
• On (Sf2KeepMismatchedStereoAsMono=1): those 3 pairs are instead kept as their 6 separate full-length mono samples (+3 files); the channel that was previously truncated keeps its full length.

Pitch- and sample-rate-mismatched pairs remain mono in both cases (unchanged).

[git-moss]

I have several examples where the left/right channels have different lengths. I suggest to add this as a destination option (which is off by default).

[douglas-carmichael]

Thanks! Updated per your suggestion: this is now an opt-in "Keep mismatched stereo samples as mono" option, off by default, so banks with genuine different-length stereo pairs keep combining exactly as before — only when it's enabled does a length mismatch keep the two samples separate (matching how a pitch or sample-rate mismatch already behaves).

Since the stereo-combining happens while reading the SoundFont (combineToStereo in the detector), I put the checkbox in the SF2 source options next to "Log unused SF2 generators" rather than on the destination side, as the destination formats never see the split-mono pair — happy to move it if you'd prefer it elsewhere.

Verified on the E-mu "Orbit" bank: off (the default) combines the 3 length-mismatched pairs as before; on keeps them as their separate full-length mono samples. Rebased onto current main.