LMMS
Fix Resized Samples getting Shifted after Tempo Change
If you change the tempo of a song, the contents of all sample clips get scaled forwards/backwards. However, they get scaled relative to the internal 0 time of the clip, not the start of the clip. This causes issues when dealing with resized or cut/sliced sample clips.
This PR fixes this issue by updating the startTimeOffset of the sample clip when the tempo changes.
Originally I was just multiplying the startTimeOffset by the ratio of the new tempo to the old tempo, but that resulted in precision errors due to integer division. Instead, I decided to store the oldstartTimeOffset in terms of frames and calculate the new startTimeOffset based on that. That way, even if the user drastically changes the bpm from 140 to 10 to 999 to 300 to 10 and back to 140, the startTimeOffset will still return to its correct value, since it's being calculated by the original frame offset.
Note
In order to store the old startTimeOffset in frames between tempo changes, I have it track with the startTimeOffset when resizing/splitting, but leaving it unaffected when changing the tempo. To do this, I overrode Clip::setStartTimeOffset in SampleClip to make it also update the frame offset. That way, resizing the clip by the user, or splitting the clip, or any other action which calls that function will update it correctly. But in the function which handles the tempo change, I still use Clip::startTimeOffset.
Before
https://github.com/user-attachments/assets/5abfcfa6-c239-4ada-bdbe-e65a37ff4ae0
After
https://github.com/user-attachments/assets/09cbe676-8d3c-4e29-979e-75cbd2c8b94c
looks good (haven't tested yet, to be clear). will this break older projects?
As far as I know it shouldn't. This PR doesn't change how samples clips are saved or loaded, and the start frame offset should always be synced with the start time offset, so the only change in behavior should happen when changing the tempo.
