Add NavigationMesh border_size property for tile baking

[smix8]

Adds NavigationMesh border_size property for tile baking.

This allows users to partition larger game worlds into regions that have navigation meshes with easy to align tile edges. This tile edges can be efficiently merged by the navigation map using edgekeys. It also avoids problems with the agent_radius offset affecting the tile edges.

The difference between baking AABB and border size is, that the AABB limits the entire source geometry used in the baking as a bounding area while the border size limits the surface of the resulting navigation mesh.

This means in conjunction they can be used to limit the final navigation mesh without having unwanted offsets by e.g. agent radius at tile edges.

When users do the setup the border_size should be larger than the agent_radius and the AABB should be grown by the exact amount of the border_size.

The border size that ReCast uses internally is in voxel units, so the Godot border size float is ceiled to the cell size.

To bake a "perfect" aligned tile navigation mesh additional properties need to be considered. E.g. the edge_max_error should be at or blow 1.0.

[akien-mga]

We merged the 2D counterpart, is this one also good to go? CC @Scony

[Scony]

All good @akien-mga

[AThousandShips]

Style and docs look good

[akien-mga]

Thanks!