matrix: fix for a corner case (e.g. gapmaps)

workaround for a corner case; if the reference pixels falls into a "gap" then gPC returns BLACK. Solutions is to reject BLACK.

wled00/FX.cpp

@@ -5207,6 +5207,7 @@ uint16_t mode_2Dmatrix(void) {                  // Matrix2D. By Jeremy Williams.
         oldSpawnColor = SEGMENT.getPixelColorXY(SEGENV.aux0, SEGENV.aux1);  // find color of previous spawns
         SEGENV.aux1 ++;                                                     // our sample pixel will be one row down the next time
     }
+    if ((oldSpawnColor == CRGB::Black) || (oldSpawnColor == trailColor)) oldSpawnColor = spawnColor; // reject "black", as it would mean that ALL pixels create trails
 
     // move pixels one row down. Falling codes keep color and add trail pixels; all others pixels are faded
     for (int row=rows-1; row>=0; row--) {