show warning when using blendMode() with PDF (fixes #5105)

java/libraries/pdf/src/processing/pdf/PGraphicsPDF.java

@@ -550,6 +550,14 @@ public class PGraphicsPDF extends PGraphicsJava2D {
 
   //
 
+  protected void blendModeImpl() {
+    if (blendMode != BLEND) {
+      nope("blendMode");
+    }
+  }
+
+  //
+
   public void copy(int sx1, int sy1, int sx2, int sy2,
                    int dx1, int dy1, int dx2, int dy2) {
     nope("copy");