Terrain3DStorage: Add get_normal() utility function

[lw64]

Fixes #142

[TokisanGames]

The original formula from the shader added _region_pixel_size to UV. The value of that is 1/1024. Here, replace _region_size with 1.0f/_region_size.

Also you can remove the tangent and binormal options, and consolidate get_normal_simple into this function. Thanks for the idea for two options, but it's not necessary.

[TokisanGames]

I updated the note above. We need make sure the offset for get_height is big enough so that it moves to the next pixel in the map image. 1/region_size should do it as it's the same size as in the shader, and region_size will change in the future.

[lw64]

This way it still didnt work as expected. I experimented a bit and came with something else, but I am not sure at all if its correct.

[TokisanGames]

I looked closer at the shader and this is how it's calculated:

get_height(uv + vec2(_region_texel_size, 0));

get_height((vertex.xz + 0.5)/1024 + vec2(1/1024, 0));

That get_height is using 0-1 UV coordinates. Our C++ get_height uses full range values, so if we multiply the above by 1024 we get vertex.xz + 0.5 + vec2(1, 0). However the 0.5 is a texel offset specifically for the shader because of the weird way glsl works, and makes no difference here (I tested it to confirm). So we have this:

	real_t left = get_height(p_global_position + Vector3(-1.0f, 0.0f, 0.0f));
	real_t right = get_height(p_global_position + Vector3(1.0f, 0.0f, 0.0f));
	real_t back = get_height(p_global_position + Vector3(0.f, 0.f, -1.0f));
	real_t front = get_height(p_global_position + Vector3(0.f, 0.f, 1.0f));

I then tested this by adding an MMI to DemoScene._ready()

	var count: int = 1024
	var mmi := MultiMeshInstance3D.new()
	mmi.multimesh = MultiMesh.new()
	mmi.multimesh.transform_format = MultiMesh.TRANSFORM_3D
	mmi.multimesh.instance_count = count*count
	mmi.multimesh.mesh = PrismMesh.new()
	mmi.multimesh.mesh.size = Vector3(.25, 1, .25)
	add_child(mmi)
	
	for x in count:
		for z in count:
			var xform := Transform3D()
			#xform.origin = Vector3(x, 0, z) * $Terrain3D.get_mesh_vertex_spacing()
			xform.origin = Vector3(x, 0, z)
			var normal: Vector3 = $Terrain3D.storage.get_normal(xform.origin)
			if is_nan(normal.x):
				normal = Vector3.UP
			normal = normal.normalized()
			xform.basis.y = normal
			xform.basis.x = -xform.basis.z.cross(normal)
			xform.basis = xform.basis.orthonormalized()
			var height: float = $Terrain3D.storage.get_height(xform.origin) 
			if is_nan(height):
				xform.origin.y = -100
			else:
				xform.origin.y = height
				xform.origin += normal * .5
			mmi.multimesh.set_instance_transform(z*count + x, xform)
                

It looks pretty good.