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/xml.cpp

@@ -505,6 +505,7 @@ void getSettingsJS(AsyncWebServerRequest* request, byte subPage, char* dest) //W
 
     sappend('c',SET_F("GB"),gammaCorrectBri);
     sappend('c',SET_F("GC"),gammaCorrectCol);
+    sappend('c',SET_F("GCP"),gammaCorrectPreview);   // WLEDMM
     dtostrf(gammaCorrectVal,3,1,nS); sappends('s',SET_F("GV"),nS);
     sappend('c',SET_F("TF"),fadeTransition);
     sappend('v',SET_F("TD"),transitionDelayDefault);