refactor stroke drawing

src/canvas.h

@@ -126,6 +126,14 @@ struct PPIHeader
 
 class Canvas
 {
+    struct StrokeFrame
+    {
+        glm::vec4 col;
+        float pressure;
+        std::array<std::vector<vertex_t>, 6> shapes;
+        glm::vec4 m_mixer_rect;
+    };
+
 public:
     Plane m_plane;
     Plane m_plane_brush;
@@ -224,6 +232,9 @@ public:
     void stroke_start(glm::vec3 point, float pressure, const std::shared_ptr<Brush>& brush);
     void stroke_update(glm::vec3 point, float pressure);
     void stroke_draw_mix(const glm::vec2& bb_min, const glm::vec2& bb_sz);
+    std::array<std::vector<vertex_t>, 6> stroke_draw_project(std::array<vertex_t, 4>& B);
+    void stroke_draw_samples(int i, std::vector<vertex_t>& P);
+    std::vector<StrokeFrame> stroke_draw_compute();
     void stroke_draw();
     void stroke_end();
     void stroke_cancel();