(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/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;
     }