Optimise fillEllipse function. ~36% faster (#478)

* Optimised fillEllipse function

* Cleaned up code

Adafruit_GFX.cpp

@@ -430,14 +430,13 @@ void Adafruit_GFX::fillEllipse(int16_t x0, int16_t y0, int16_t rw, int16_t rh,
 
   // region 1
   while ((twoRh2 * x) < (twoRw2 * y)) {
-    drawFastHLine(x0 - x, y0 + y, 2 * x + 1, color);
-    drawFastHLine(x0 - x, y0 - y, 2 * x + 1, color);
-
     x++;
     if (decision < 0) {
       decision += rh2 + (twoRh2 * x);
     } else {
       decision += rh2 + (twoRh2 * x) - (twoRw2 * y);
+      drawFastHLine(x0 - (x - 1), y0 + y, 2 * (x - 1) + 1, color);
+      drawFastHLine(x0 - (x - 1), y0 - y, 2 * (x - 1) + 1, color);
       y--;
     }
   }