improved FPS calc resolution, added averaging & multiplier compileflags

Fixed point calculation for improved accuracy, dithering in debug builds only.
Averaging and optional multiplier can be set as compile flags, example for speed testing with long averaging and a 10x multiplier:

-D FPS_CALC_AVG=200
-D FPS_MULTIPLIER=10

The calculation resolution is limited (9.7bit fixed point) so values larger than 200 can hit resolution limit and get stuck before reaching the final value.

If WLED_DEBUG is defined, dithering is added to the returned value so sub-frame accuracy is possible in post-processingwithout enabling the multiplier.

wled00/FX.h

@@ -47,6 +47,14 @@
 #define FRAMETIME_FIXED  (1000/WLED_FPS)
 #define FRAMETIME        strip.getFrameTime()
 
+// FPS calculation (can be defined as compile flag for debugging)
+#ifndef FPS_CALC_AVG
+#define FPS_CALC_AVG 7 // average FPS calculation over this many frames (moving average)
+#endif
+#ifndef FPS_MULTIPLIER
+#define FPS_MULTIPLIER 1 // dev option: multiplier to get sub-frame FPS without floats
+#endif
+
 /* each segment uses 82 bytes of SRAM memory, so if you're application fails because of
   insufficient memory, decreasing MAX_NUM_SEGMENTS may help */
 #ifdef ESP8266
@@ -729,7 +737,7 @@ class WS2812FX {  // 96 bytes
       _transitionDur(750),
       _targetFps(WLED_FPS),
       _frametime(FRAMETIME_FIXED),
-      _cumulativeFps(2),
+      _cumulativeFps(50<<6),
       _isServicing(false),
       _isOffRefreshRequired(false),
       _hasWhiteChannel(false),