Two fixes:

- when rendering a scene, all composite trees of other trees should get
  signalled the render output changed. This only happened with a composite
  active in render.

- the Mix node "A" option only works in Composite, as accidentally visible
  for shader trees.

source/blender/render/intern/source/pipeline.c

@@ -2062,9 +2062,6 @@ static int is_rendering_allowed(Render *re)
 				re->error("No Nodetree in Scene");
 				return 0;
 			}
-		
-			/* do it here, so stack gets freed on esc */
-			ntreeCompositTagRender(re->scene);
 			
 			for(node= ntree->nodes.first; node; node= node->next)
 				if(node->type==CMP_NODE_COMPOSITE)
@@ -2158,6 +2155,9 @@ static int render_initialize_from_scene(Render *re, Scene *scene)
 	
 	RE_InitState(re, &scene->r, winx, winy, &disprect);
 	
+	/* initstate makes new result, have to send changed tags around */
+	ntreeCompositTagRender(re->scene);
+	
 	re->scene= scene;
 	if(!is_rendering_allowed(re))
 		return 0;

source/blender/src/drawnode.c

@@ -273,15 +273,17 @@ static int node_buts_mix_rgb(uiBlock *block, bNodeTree *ntree, bNode *node, rctf
 {
 	if(block) {
 		uiBut *bt;
+		int a_but= (ntree->type==NTREE_COMPOSIT);
 		
 		/* blend type */
 		uiBlockBeginAlign(block);
 		bt=uiDefButS(block, MENU, B_NODE_EXEC+node->nr, "Mix %x0|Add %x1|Subtract %x3|Multiply %x2|Screen %x4|Overlay %x9|Divide %x5|Difference %x6|Darken %x7|Lighten %x8|Dodge %x10|Burn %x11|Color %x15|Value %x14|Saturation %x13|Hue %x12",
-					 butr->xmin, butr->ymin, butr->xmax-butr->xmin-20, 20, 
+					 butr->xmin, butr->ymin, butr->xmax-butr->xmin -(a_but?20:0), 20, 
 					 &node->custom1, 0, 0, 0, 0, "");
 		uiButSetFunc(bt, node_but_title_cb, node, bt);
-		/* Alpha option */
-		uiDefButS(block, TOG, B_NODE_EXEC+node->nr, "A",
+		/* Alpha option, composite */
+		if(a_but)
+			uiDefButS(block, TOG, B_NODE_EXEC+node->nr, "A",
 				  butr->xmax-20, butr->ymin, 20, 20, 
 				  &node->custom2, 0, 0, 0, 0, "Include Alpha of 2nd input in this operation");
 	}