Add means for fixing navmap synchronization errors

[Scony]

Fixes #85548

This PR is based on my observation that for some valid input geometries, recast produces valid navigation meshes and yet the transformation that is done as part vertex hashing:

godot/modules/navigation/nav_map.cpp

Lines 106 to 108 in b4e2a24

	const int x = static_cast<int>(Math::floor(p_pos.x / cell_size));
	const int y = static_cast<int>(Math::floor(p_pos.y / cell_height));
	const int z = static_cast<int>(Math::floor(p_pos.z / cell_size));

is making those navigation meshes invalid. That is because some vertices are conflicting with others after transformation above and thus leading to synchronization errors mentioned in #85548

This PR proposes a map-level option to change the scale of internal rasterizer which allows the engine to perform the align-to-recast-grid transformation using greater precision. With that, in projects where navigation baking leads to synchronization problems, the precision of internal rasterizer can be increased to avoid errors (although potentially sacrificing the ability to connect separate regions).

I've prepared a straightforward demo (godot-navigation-demos.zip - ComplexNavmeshPaths.tscn) application that showcases the problem - all it does is query the server for paths from random point A to random point B each and render them to show potential holes in navigation.

The demo outcome when using current Godot implementation:

The demo outcome when the Godot from this PR is used and merge_rasterizer_cell_scale is set to 0.001:

[Mickeon]

Docs look good enough, I just cannot speak on the feature itself.

[akien-mga]

Thanks!