Improve NavMeshGenerator2D::generator_bake_from_source_geometry_data performance

[kiroxas]

Removed a lot of unecessary copies and duplicated work in NavMeshGenerator2D::generator_bake_from_source_geometry_data.

Tested on all godot examples project 2D Navigation. (navigation, navigation_astar, navigation_mesh_chunks)
Reduced navmesh creation for the chunk project from 34 000 000 cycles to 17 000 000 cycles on average. (mesured with RDTSC)
So roughly a 2x speed up.

[smix8]

Agree that the 2d baking could be improved and some of the changes here look fine.
Although making the internal source geometry arrays and locks public can't be done. This needs to be handled differently.

[kiroxas]

We can probably get away with it with some getter returning const references. They exist (at least two of them) but they look wrong to me :
const Vector<Vector<Vector2>> &NavigationMeshSourceGeometryData2D::_get_traversable_outlines() const { RWLockRead read_lock(geometry_rwlock); return traversable_outlines; }
as the lock here is not locked when accessing the data later, which makes it wrong to use this instead of what I'm doing. But I can totally use them and create another one for _get_projected_obstructions, if this is the preferred way.

[kiroxas]

So the performance is roughly the same with the getters, so I made the change. Only the function is changed now.

[kiroxas]

As suggested by smix8, I took the friend path instead of public.

[smix8]

Tested a few projects and bakes fine without any noticeable error.

Still needs a code review.

[akien-mga]

Just pushed an amend to edit the commit message to match the PR title and be more explicit.

[Repiteo]

Thanks! Congratulations on your first contribution! 🎉