attach jni to ui thread, fix bucket bounding box

src/canvas.cpp

@@ -1395,7 +1395,8 @@ void Canvas::flood_fill(int layer, int plane, std::vector<glm::ivec2> pos, Flood
                     rgb[i] = dest_color * 255.f;
                     plane_data.dirty[plane] = true;
                     glm::vec2 bb_min = glm::min((glm::vec2)p, xy(plane_data.bb[plane]));
-                    glm::vec2 bb_max = glm::max((glm::vec2)p, zw(plane_data.bb[plane]));
+                    // add 1 pixel to the end because 1 pixel has min(0) and max(1)
+                    glm::vec2 bb_max = glm::max((glm::vec2)p + glm::vec2(1), zw(plane_data.bb[plane]));
                     plane_data.bb[plane] = { bb_min, bb_max };
                 }
                 pos.push_back(p);