mixer working pretty well now

engine/canvas.h

@@ -90,6 +90,7 @@ public:
     static glm::mat4 m_plane_transform[6];
     glm::vec2 stencil_offset;
     Sampler m_sampler;
+    Sampler m_sampler_linear;
     Sampler m_sampler_brush;
     Sampler m_sampler_bg;
     Sampler m_sampler_mask;
@@ -131,7 +132,7 @@ public:
     void layer_merge(int source_idx, int dest_idx);
     void stroke_start(glm::vec2 point, float pressure, const ui::Brush& brush);
     void stroke_update(glm::vec2 point, float pressure);
-    void stroke_draw_mix();
+    void stroke_draw_mix(const glm::vec2& bb_min, const glm::vec2& bb_sz);
     void stroke_draw();
     void stroke_end();
     void stroke_cancel();