Subtle corrections for NORM_MIN

This is used all over and should be the same for OpenCL and CPU code for all modules.
It's now OpenCL-public via kernels/common.h

data/kernels/channelmixer.cl

@@ -41,7 +41,6 @@ typedef enum dt_adaptation_t
 #define INVERSE_SQRT_3 0.5773502691896258f
 #define TRUE 1
 #define FALSE 0
-#define NORM_MIN 1e-6f
 
 static inline float sqf(const float x)
 {

data/kernels/common.h

@@ -17,6 +17,8 @@
 */
 
 #pragma once
+#define NORM_MIN 1.52587890625e-05f // norm can't be < to 2^(-16)
+
 
 constant sampler_t sampleri =  CLK_NORMALIZED_COORDS_FALSE | CLK_ADDRESS_CLAMP_TO_EDGE | CLK_FILTER_NEAREST;
 

data/kernels/filmic.cl

@@ -291,9 +291,6 @@ static inline float filmic_spline(const float x,
   return result;
 }
 
-#define NORM_MIN 1.52587890625e-05f // norm can't be < to 2^(-16)
-
-
 static inline float log_tonemapping_v1(const float x,
                                        const float grey, const float black,
                                        const float dynamic_range)