Fix span modifier can be overridden

Author: cheonjaeung

grid/src/commonMain/kotlin/com/cheonjaeung/compose/grid/BoxGridSpanModifier.kt

@@ -39,8 +39,8 @@ internal class BoxGridSpanNode(
 ) : Modifier.Node(), ParentDataModifierNode {
     override fun Density.modifyParentData(parentData: Any?): Any {
         val p = parentData as? BoxGridParentData ?: BoxGridParentData()
-        span?.let {
-            p.span = it
+        if (p.span == null) {
+            p.span = span
         }
         return p
     }

grid/src/commonMain/kotlin/com/cheonjaeung/compose/grid/HorizontalVerticalGridSpanModifier.kt

@@ -39,7 +39,9 @@ internal class HorizontalVerticalGridSpanNode(
 ) : Modifier.Node(), ParentDataModifierNode {
     override fun Density.modifyParentData(parentData: Any?): Any {
         val p = parentData as? HorizontalVerticalGridParentData ?: HorizontalVerticalGridParentData()
-        p.span = span
+        if (p.span == HorizontalVerticalGridParentData.DefaultSpan) {
+            p.span = span
+        }
         return p
     }
 }