Check if symbol is function before calling symbolNumber() in backoff calculation

Co-authored-by: gatopeich <7722268+gatopeich@users.noreply.github.com>

Author: Copilot

src/components/drawing/index.js

@@ -1506,7 +1506,12 @@ function applyBackoff(pt, start) {
             var endMarkerSize = endMarker.size;
             if (Lib.isArrayOrTypedArray(endMarkerSize)) endMarkerSize = endMarkerSize[endI];
 
-            b = endMarker ? (drawing.symbolBackOffs[drawing.symbolNumber(endMarkerSymbol)] || 0) * endMarkerSize : 0;
+            // Custom functions don't have backoff values, only built-in symbols do
+            var backoffValue = 0;
+            if (endMarker && typeof endMarkerSymbol !== 'function') {
+                backoffValue = drawing.symbolBackOffs[drawing.symbolNumber(endMarkerSymbol)] || 0;
+            }
+            b = backoffValue * endMarkerSize;
             b += drawing.getMarkerStandoff(d[endI], trace) || 0;
         }