Screen blend mode for andriod.

jme3-android/src/main/java/com/jme3/renderer/android/OGLESShaderRenderer.java

@@ -556,6 +556,9 @@ public class OGLESShaderRenderer implements Renderer {
                     case ModulateX2:
                         GLES20.glBlendFunc(GLES20.GL_DST_COLOR, GLES20.GL_SRC_COLOR);
                         break;
+                    case Screen:
+                        GLES20.glBlendFunc(GLES20.GL_ONE, GLES20.GL_ONE_MINUS_SRC_COLOR);
+                        break; 
                     default:
                         throw new UnsupportedOperationException("Unrecognized blend mode: "
                                 + state.getBlendMode());