fix stencil random every stroke and not sample

engine/canvas.h

@@ -80,6 +80,7 @@ public:
     static glm::vec3 m_plane_normal[6];
     static glm::vec3 m_plane_tangent[6];
     static glm::mat4 m_plane_transform[6];
+    glm::vec2 stencil_offset;
     Sampler m_sampler;
     Sampler m_sampler_brush;
     Sampler m_sampler_bg;