implement flood fill algorithm

src/canvas.h

@@ -166,6 +166,8 @@ public:
     void layer_add(std::string name, std::shared_ptr<Layer> layer = nullptr, int index = 0);
     void layer_order(int idx, int pos);
     void layer_merge(int source_idx, int dest_idx);
+    void flood_fill(int layer, int plane, std::vector<glm::ivec2> pos, std::map<int, std::unique_ptr<bool[]>>& plane_mask,
+        float threshold, glm::vec4 dest_color, std::unique_ptr<glm::vec4>& source_color);
     void stroke_start(glm::vec3 point, float pressure);
     void stroke_update(glm::vec3 point, float pressure);
     void stroke_draw_mix(const glm::vec2& bb_min, const glm::vec2& bb_sz);