Tweak/code shuffle to make disabled buttons draw nicer on nodes.

source/blender/editors/interface/interface.c

@@ -749,8 +749,6 @@ void uiDrawBlock(const bContext *C, uiBlock *block)
 	
 	wmOrtho2(-0.01f, ar->winx-0.01f, -0.01f, ar->winy-0.01f);
 	
-	UI_ThemeClearColor(TH_BACK);
-	
 	/* back */
 	if(block->flag & UI_BLOCK_LOOP)
 		ui_draw_menu_back(&style, block, &rect);

source/blender/editors/interface/interface_panel.c

@@ -864,6 +864,8 @@ void uiEndPanels(const bContext *C, ARegion *ar)
 	if(firstpa)
 		firstpa->runtime_flag |= PNL_FIRST;
 
+	UI_ThemeClearColor(TH_BACK);
+	
 	/* draw panels, selected on top */
 	for(block= ar->uiblocks.first; block; block=block->next) {
 		if(block->active && block->panel && !(block->panel->flag & PNL_SELECT)) {

source/blender/editors/space_node/node_draw.c

@@ -845,6 +845,8 @@ static void node_draw_basis(const bContext *C, ARegion *ar, SpaceNode *snode, bN
 			node_draw_preview(node->preview, &node->prvr);
 		BLI_unlock_thread(LOCK_PREVIEW);
 	}
+	
+	UI_ThemeClearColor(color_id);
 		
 	uiEndBlock(C, node->block);
 	uiDrawBlock(C, node->block);