Fixing greater-than-2-channel audio clip waveform drawing leads to instability

With commit 44386572c543f6332ce85bce431e9338e6e41534 and later Qtractor crashes sometimes when scrolling in timeline, see attached screencast. The faster the scrolling the higher the crash probability. Maybe some race condition? [simplescreenrecorder-2025-07-08_11.42.15.mkv.zip](https://github.com/user-attachments/files/21119197/simplescreenrecorder-2025-07-08_11.42.15.mkv.zip)