fix mixer clear residual data, reduce brush preview updates especially on gesture

src/canvas.cpp

@@ -1259,6 +1259,7 @@ void Canvas::stroke_start(glm::vec3 point, float pressure)
             m_tmp_dual[i].unbindFramebuffer();
         }
     }
+    m_mixer.clear();
     m_show_tmp = true;
 }
 void Canvas::layer_add(std::string name, std::shared_ptr<Layer> layer /*= nullptr*/, int index /*= 0*/)
@@ -1670,7 +1671,6 @@ bool Canvas::create(int width, int height)
     m_plane.create<1>(1, 1);
     m_plane_brush.create<1>(1, 1);
     m_brush_shape.create();
-    m_brush_mix.create(8, 8);
     for (auto& l : m_layers)
         l->create(width, height, "");
     m_smask.create(width, height, "mask");