Add Screen blend mode.

jme3-core/src/main/java/com/jme3/material/RenderState.java

@@ -176,7 +176,14 @@ public class RenderState implements Cloneable, Savable {
          * <p>
          * Result = 2 * Source Color * Dest Color -> (GL_DST_COLOR, GL_SRC_COLOR)
          */
-        ModulateX2
+        ModulateX2,
+        /**
+         * Opposite effect of Modulate/Multiply. Invert both colors, multiply and
+         * then invert the result.
+         * <p>
+         * Result = 2 * Source Color * Dest Color -> (GL_ONE, GL_ONE_MINUS_SRC_COLOR)
+         */
+        Screen
     }
 
     /**

jme3-lwjgl/src/main/java/com/jme3/renderer/lwjgl/LwjglRenderer.java

@@ -647,6 +647,9 @@ public class LwjglRenderer implements Renderer {
                     case ModulateX2:
                         glBlendFunc(GL_DST_COLOR, GL_SRC_COLOR);
                         break;
+                    case Screen:
+                        glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_COLOR);
+                        break;       
                     default:
                         throw new UnsupportedOperationException("Unrecognized blend mode: "
                                 + state.getBlendMode());