fix bucket undo

src/canvas.h

@@ -71,7 +71,7 @@ public:
         std::shared_ptr<Layer> layer;
         std::unique_ptr<bool[]> mask[6];
         std::unique_ptr<glm::u8vec4[]> rgb[6] = SIXPLETTE(0);
-        bool dirty[6] = SIXPLETTE(false);
+        std::array<bool, 6> dirty = SIXPLETTE(false);
         glm::vec4 bb[6];
         void apply();
     };