device/kobo/sysfs_light: Make used exponent in formulas a variable

frontend/device/kobo/sysfs_light.lua

@@ -17,6 +17,7 @@ local KoboSysfsLight = {
     white_offset = -25,
     red_offset = 0,
     green_offset = -65,
+    exponent = 0.25,
 }
 
 function KoboSysfsLight:new(o)
@@ -61,14 +62,14 @@ function KoboSysfsLight:setNaturalBrightness(brightness, warmth)
     if brightness > 0 then
         -- On Nickel, the values for white/red/green are roughly linearly dependent
         -- on the 4th root of brightness and warmth.
-        white = math.min(self.white_gain * math.pow(brightness, 0.25) *
-                             math.pow(100 - warmth, 0.25) + self.white_offset, 255)
+        white = math.min(self.white_gain * math.pow(brightness, self.exponent) *
+                             math.pow(100 - warmth, self.exponent) + self.white_offset, 255)
     end
     if warmth > 0 then
-        red = math.min(self.red_gain * math.pow(brightness, 0.25) *
-                           math.pow(warmth, 0.25) + self.red_offset, 255)
-        green = math.min(self.green_gain * math.pow(brightness, 0.25) *
-                             math.pow(warmth, 0.25) + self.green_offset, 255)
+        red = math.min(self.red_gain * math.pow(brightness, self.exponent) *
+                           math.pow(warmth, self.exponent) + self.red_offset, 255)
+        green = math.min(self.green_gain * math.pow(brightness, self.exponent) *
+                             math.pow(warmth, self.exponent) + self.green_offset, 255)
     end
 
     white = math.max(white, 0)