VERSION 0.121.1371: realtime-renderer stability improvement and more

I now solved the problem, that some fractals fade to black during editing or realtime-animation-preview.

And, as usual, there is much more, see the full changelog:
VERSION 0.121.1371 (15.09.2025):
- realtime-animation-preview and realtime-editing: the renderer automatically
analyzes the iteration-buffers and performs a re-initialization when
too many samples are turning to infinite/invalid numbers (this effect causes
the fractal to get darker with time and happens when the fractal reaches
an unstable (in-between) state during editing or animation) (SWAN-59)
- flame-editor: clone also the variation-priority when duplicating a variation (SWAN-153)
- flame-editor: show also the time in realtime-animation-preview
- motion-curve-modulation: added an additional sum_fft() which accumulates a range
of fft-channels, because it is sometimes difficult to pick the right one if you
can only pick one
- motion-curve-editor: the frame-count for stretching key-frames is now initialized
with the flame's frame-count, to avoid confusion (SWAN-95)
- motion-curve-editor: stop all audio when closing the window
- main editor: random-motion-function: keep the frame-count of the flame if it larger
than (2 *number of key frames to generate) (SWAN-151)
- motion-curve-editor: take also the frame-count into account when calculating
the motion-curve-view-bounds (SWAN-151)
- motion-curve-editor: play from current frame position now also works when
playing with sound (and synchronizing frames to the audio)
- made the borders of sub-windows more consistent
- some optimizations of the animation-render-window, regarding rendering animations
with many frames
- main-editor: fixed a bug regarding editing key-frames with negative frame numbers
- application-settings: introduces boolean parameters which are represended by a
check button instead of a text field (to enter "0" or "1")
- new "ringer"-variation, created by Rick Sidwell