HUB75 cleanup, double buffer improvement

* code cleanup, removing some unneeded sanity checks
* use dirty bits in double buffer mode, too -> only not-black pixels are drawn into the back-buffer (20%-30% faster)

wled00/bus_manager.h

@@ -389,7 +389,6 @@ class BusHub75Matrix : public Bus {
     MatrixPanel_I2S_DMA *display = nullptr;
     VirtualMatrixPanel  *fourScanPanel = nullptr;
     HUB75_I2S_CFG mxconfig;
-    bool isBlack = false;
     CRGB *_ledBuffer = nullptr;
     byte *_ledsDirty = nullptr;
 };