Fix bug where global planner lethal cost is always 1 unit smaller than expected

[siferati]

Description

• Fix [Bug] [Global Planner] Lethal cost is always 1 unit smaller than user setting #1093

Global planner treats lethal_cost_ as if it was the costmap_2d::LETHAL_OBSTACLE cost, and so assumes that lethal_cost_ - 1 is also in collision (i.e. costmap_2d::INSCRIBED_INFLATED_OBSTACLE).

The whole point of this param being dynamically reconfigured is that we don't have to be tied to the value of costmap_2d::LETHAL_OBSTACLE and so we also shouldn't assume that lethal_cost_ - 1 is lethal too.
That's also why the default value for lethal_cost_ is 253, not 254.

navigation/global_planner/cfg/GlobalPlanner.cfg

Line 8 in 9ad6441

gen.add("lethal_cost", int_t, 0, "Lethal Cost", 253, 1, 255)

Because when planning a path, a cell of cost 253 (i.e. inscribed) is indeed a lethal cell the robot can not traverse.
However, as it stands, the global planner will also treat cells of cost 252 as lethal, which is obviously wrong.