minor optimization of core LED functions

* use _fast_ integer types in loops - in contrast to "uint16_t" etc, the compiler can leave out range/overflow corrections, so it might run faster.
* fcn_declare.h: revive "WLED_USE_REAL_MATH" option, which can be a bit faster on ESP32.

wled00/led.cpp

@@ -70,7 +70,8 @@ void toggleOnOff()
 //scales the brightness with the briMultiplier factor
 byte scaledBri(byte in)
 {
-  uint16_t val = ((uint16_t)in*briMultiplier)/100;
+  if (briMultiplier == 100) return(in); // WLEDMM shortcut
+  uint_fast16_t val = ((uint_fast16_t)in*(uint_fast16_t)briMultiplier)/100; // WLEDMM
   if (val > 255) val = 255;
   return (byte)val;
 }