bugfix for holes in 2D DNA Spiral

Holes were visible at height > 32. Root cause: "lerp8x8" seems to be inaccurate --> replaced by a simple linear calculation.

wled00/FX.cpp

@@ -5010,9 +5010,11 @@ uint16_t mode_2DDNASpiral() {               // By: ldirko  https://editor.soulma
       // draw a gradient line between x and x1
       x = x / 2; x1 = x1 / 2;
       unsigned steps = abs8(x - x1) + 1;
+      bool positive = (x1 >= x);                         // direction of drawing
       for (size_t k = 1; k <= steps; k++) {
         unsigned rate = k * 255 / steps;
-        unsigned dx = lerp8by8(x, x1, rate);
+        //unsigned dx = lerp8by8(x, x1, rate);
+        unsigned dx = positive? (x + k-1) : (x - k+1);   // behaves the same as "lerp8by8" but does not create holes
         //SEGMENT.setPixelColorXY(dx, i, ColorFromPalette(SEGPALETTE, hue, 255, LINEARBLEND).nscale8_video(rate));
         SEGMENT.addPixelColorXY(dx, i, ColorFromPalette(SEGPALETTE, hue, 255, LINEARBLEND)); // use setPixelColorXY for different look
         SEGMENT.fadePixelColorXY(dx, i, rate);