code robustness improvements plus minor speedup

* make  XY() and _setPixelColorXY_raw() const (minor speedup)
* segment is a struct not a class: friend class Segment --> friend struct Segment
* fix missing braces around two macros
* use non-throwing "new" where possible
* improve robustness of transition code

wled00/playlist.cpp

@@ -61,7 +61,7 @@ int16_t loadPlaylist(JsonObject playlistObj, byte presetId) {
   if (playlistLen == 0) return -1;
   if (playlistLen > 100) playlistLen = 100;
 
-  playlistEntries = new PlaylistEntry[playlistLen];
+  playlistEntries = new(std::nothrow) PlaylistEntry[playlistLen];
   if (playlistEntries == nullptr) return -1;
 
   byte it = 0;