FilmNeg tool: used a single luminance reference when sampling white balance spot several times in sequence, to avoid luminance drift.

Fixes #6257

rtgui/filmnegative.h

@@ -99,6 +99,17 @@ private:
     RGB refInputValues;
     bool paramsUpgraded;
 
+    /**
+     * Luminance reference: these input RGB values must produce this output luminance.
+     * This allows keeping constant luminance when picking several white balance  spot
+     * samples in sequence; values are set on the initial white balance spot sampling,
+     * and reset every time params are read, or the output sliders are changed.
+     */
+    struct {
+        RGB input;
+        float lum;
+    } refLuminance;
+
     FilmNegProvider* fnp;
 
     MyComboBoxText* const colorSpace;