(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]]

wled00/ws.cpp

@@ -290,7 +290,7 @@ static bool sendLiveLedsWs(uint32_t wsClient)  // WLEDMM added "static"
   #endif
 
   (void) unGamma8(127); // WLEDMM dummy call, just to make sure that gammaTinv is initialized, so we can use fast_unGamma8
-  uint8_t stripBrightness = strip.getBrightness();
+  // [[maybe_unused]] uint8_t stripBrightness = strip.getBrightness();
   for (size_t i = 0; pos < bufSize -2; i += n)
   {
   //WLEDMM skipping lines done right