Luma scale adjustment does not change image brightness

[for13to1]

Describe the bug
When adjusting the Luma scale slider, the image brightness does not change. Only contrast changes (brighter pixels get brighter, darker pixels get darker), while overall average brightness remains the same.

To Reproduce

1. Open any YUV file
2. Locate the Luma scale slider in the properties panel
3. Increase or decrease the Luma scale value
4. Observe that overall brightness does not change noticeably

Expected behavior
Adjusting Luma scale should produce visible brightness changes across the entire image.

Root Cause
The default Luma offset is set to 125 (limited range center). The transform formula:

$$newValue = (value - offset) * scale + offset$$

When offset = 125, pixels near this value are minimally affected by scale changes.

Version

• OS: macOS
• Version: latest develop branch

[for13to1]

#679