experimental: preview with gamma correction

.... actually we undo gamma adjustments, because screens (laptop, pad, etc) will apply their own gamma corrections.

Activate in LED settings: "Use Gamma correction for preview"

Colors in WLED are sometimes gamma corrected, sometimes not. This change tries to make the best out of the color mess, but its still not working properly in all configurations.

wled00/fcn_declare.h

@@ -67,6 +67,7 @@ uint8_t gamma8_cal(uint8_t b, float gamma);
 void calcGammaTable(float gamma);
 uint8_t __attribute__((pure)) gamma8(uint8_t b);                                              // WLEDMM: added attribute pure
 uint32_t __attribute__((pure)) gamma32(uint32_t);                                             // WLEDMM: added attribute pure
+uint8_t unGamma8(uint8_t value);                                                              // WLEDMM revert gamma correction
 
 //dmx.cpp
 void initDMX();