(chores) cleanup some compiler warnings

* switch-cases that fall through
* potential buffer overflow in improv.cpp
* potentially uninitialized variables in FX.cpp
* potential array out-of-range on segment::col[]
* minor optimization: only apply gamma correction when result is needed
* tag some possibly unused variables with   [[maybe_unused]]

usermods/audioreactive/audio_reactive.h

@@ -176,7 +176,7 @@ static bool limiterOn = false;                 // bool: enable / disable dynamic
 #else
 static bool limiterOn = true;
 #endif
-static uint8_t micQuality = 0;   // affects input filtering; 0 normal, 1 minimal filtering, 2 no filtering
+[[maybe_unused]] static uint8_t micQuality = 0;   // input filtering; 0 normal, 1 minimal filtering, 2 no filtering - unused on 8266
 #ifdef FFT_USE_SLIDING_WINDOW
 static uint16_t attackTime = 24;              // int: attack time in milliseconds. Default 0.024sec
 static uint16_t decayTime = 250;              // int: decay time in milliseconds.  New default 250ms.

usermods/usermod_v2_rotary_encoder_ui_ALT/usermod_v2_rotary_encoder_ui_ALT.h

@@ -395,6 +395,7 @@ const char **RotaryEncoderUIUsermod::re_findModeStrings(const char json[], int n
         break;
       case ',':
         if (!insideQuotes) modeIndex++;
+         // falls through
       default:
         if (!insideQuotes) break;
     }