Add blend mode that sums alphas and does alpha blending for colors

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

@@ -303,6 +303,17 @@ public class RenderState implements Cloneable, Savable {
          *          (1 - Source Alpha) * Dest Color -> (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)
          */
         Alpha,
+         /**
+         * Alpha blending, interpolates to source color from dest color
+         * using source alpha.
+         * The resulting alpha is the sum between the source alpha and the destination alpha.
+         * <p>
+         * Result.rgb = Source Alpha * Source Color +
+         *          (1 - Source Alpha) * Dest Color -> (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)
+         * Result.a = 1 * Source Alpha + 1 * Dest Alpha -> (GL_ONE, GL_ONE)
+         * 
+         */
+        AlphaSumA,
         /**
          * Multiplies the source and dest colors.
          * <p>

jme3-core/src/main/java/com/jme3/renderer/opengl/GLRenderer.java

@@ -789,6 +789,14 @@ public final class GLRenderer implements Renderer {
                 case Alpha:
                     blendFunc(RenderState.BlendFunc.Src_Alpha, RenderState.BlendFunc.One_Minus_Src_Alpha);
                     break;
+                case AlphaSumA:
+                    blendFuncSeparate(
+                        RenderState.BlendFunc.Src_Alpha, 
+                        RenderState.BlendFunc.One_Minus_Src_Alpha,
+                        RenderState.BlendFunc.One,
+                        RenderState.BlendFunc.One                    
+                    );
+                    break;
                 case PremultAlpha:
                     blendFunc(RenderState.BlendFunc.One, RenderState.BlendFunc.One_Minus_Src_Alpha);
                     break;