implement perspective correct interpolation to the 2D window space clipped brush stroke sample.

src/canvas.h

@@ -105,6 +105,7 @@ class Canvas
 public:
     Plane m_plane;
     Plane m_plane_brush;
+    DynamicShape m_brush_shape;
     BrushMesh m_mesh;
     bool m_unsaved = false;
     bool m_newdoc = true;
@@ -129,6 +130,8 @@ public:
     bool m_show_tmp = false;
     std::vector<Layer> m_layers;
     std::vector<int> m_order;
+    glm::mat4 m_plane_unproject[6] = SIXPLETTE(glm::mat4(1));
+    glm::vec3 m_plane_dir[6] = SIXPLETTE(glm::vec3(0));
     glm::vec4 m_dirty_box[6] = SIXPLETTE(glm::vec4(0));
     bool m_dirty_face[6] = SIXPLETTE(false);
     Layer m_smask; // selection mask