Avoid colors tending toward black when smudging

By adding a fudge factor. Not too sure why it happens in the first
place, since we round things properly, but this seems to be a fix for
anything going wrong in that regard. Maybe just an artifact of the
algorithm or caused by the inaccuracy of the 8 bit protocol that we have
to compensate for.

extern/drawdance/libengine/dpengine/layer_content.c

@@ -415,9 +415,12 @@ static DP_UPixelFloat sample_dab_color(DP_LayerContent *lc, DP_BrushStamp stamp)
     alpha /= weight;
 
     // Unpremultiply, clamp against rounding error.
-    red = CLAMP(red / alpha, 0.0f, 1.0f);
-    green = CLAMP(green / alpha, 0.0f, 1.0f);
-    blue = CLAMP(blue / alpha, 0.0f, 1.0f);
+    // The fudge factor avoids smudging causing the color to tend toward black.
+    // Not entirely sure why that happens, but fudging fixes it, so whatever.
+    float fudge = 0.005f;
+    red = CLAMP(red / alpha + fudge, 0.0f, 1.0f);
+    green = CLAMP(green / alpha + fudge, 0.0f, 1.0f);
+    blue = CLAMP(blue / alpha + fudge, 0.0f, 1.0f);
 
     return (DP_UPixelFloat){
         .b = blue,