Fix Segment::fade_out for 2d expansion

Since we're using get/setPixelColorRaw, we need the true length, not
the virtual length.

Author: willmmiles

wled00/FX_fcn.cpp

@@ -989,7 +989,8 @@ void Segment::fade_out(uint8_t rate) const {
   if (!isActive()) return; // not active
   rate = (256-rate) >> 1;
   const int mappedRate = 256 / (rate + 1);
-  for (unsigned j = 0; j < vLength(); j++) {
+  const size_t length = is2D() ? (vWidth() * vHeight()) : vLength();
+  for (unsigned j = 0; j < length; j++) {
     uint32_t color = getPixelColorRaw(j);
     if (color == colors[1]) continue; // already at target color
     for (int i = 0; i < 32; i += 8) {