renderer: fix dash offset

The dash offset value was not copied while
'=' opetator was applied.

@Issue: https://github.com/thorvg/thorvg/issues/1643

src/renderer/tvgRender.h

@@ -129,6 +129,7 @@ struct RenderStroke
             dashPattern = nullptr;
         }
         dashCnt = rhs.dashCnt;
+        dashOffset = rhs.dashOffset;
         miterlimit = rhs.miterlimit;
         cap = rhs.cap;
         join = rhs.join;