add ComboBox node, add blend modes button in stroke panel, move brush shader code to ShaderManager and avoid the same shader being recompiled many times

engine/layout.h

@@ -26,6 +26,11 @@ public:
         auto i = m_layouts.find(id);
         return i == m_layouts.end() ? nullptr : i->second.get();
     }
+    class Node* get(uint16_t id)
+    {
+        auto i = m_layouts.find(id);
+        return i == m_layouts.end() ? nullptr : i->second.get();
+    }
     void restore_context();
     void clear_context();
     //Node& operator[](const char* ids) { return m_layouts[const_hash(ids)]; }