Solve #2136 (flow-control issue with glTF alpha parameter) (#2138)

* Fix flow control issue with alpha parameter

* Revert "Fix flow control issue with alpha parameter"

This reverts commit 3ee3473.

* Fix flow control issue with alpha parameter

Author: tonihele

jme3-plugins/src/gltf/java/com/jme3/scene/plugins/gltf/PBRMaterialAdapter.java

@@ -73,9 +73,11 @@ protected MatParam adaptMatParam(MatParam param) {
                     break;
                 case "BLEND":
                     getMaterial().getAdditionalRenderState().setBlendMode(RenderState.BlendMode.Alpha);
-                    // Alpha is a RenderState not a Material Parameter, so return null
-                    return null;
+                    break;
             }
+
+            // Alpha mode is handled here, so return null
+            return null;
         } else if (param.getName().equals("doubleSided")) {
             boolean doubleSided = (boolean) param.getValue();
             if (doubleSided) {