fixed spotlight shadow issue due to incorrect basis reconstruction

[JeroenHoogers]

Objective

Fixes #24232

Solution

The basis vector construction code in the shader was no longer in sync with the rust code. I updated the shader to match the rust code:

bevy/crates/bevy_light/src/spot_light.rs

Lines 172 to 183 in 2002a18

	pub fn orthonormalize(z_basis: Dir3) -> Mat3 {
	let sign = 1f32.copysign(z_basis.z);
	let a = -1.0 / (sign + z_basis.z);
	let b = z_basis.x * z_basis.y * a;
	let x_basis = Vec3::new(
	1.0 + sign * z_basis.x * z_basis.x * a,
	sign * b,
	-sign * z_basis.x,
	);
	let y_basis = Vec3::new(b, sign + z_basis.y * z_basis.y * a, -z_basis.y);
	Mat3::from_cols(x_basis, y_basis, z_basis.into())
	}

Testing

Run the example code posted in #24232

Showcase

Before:

After:

[kfc35]

might be a dumb question, but is there a reason why this can't delegate to orthonormalize in bevy_render::maths? to reduce on the number of places that this logic should be kept in sync. that function also has a reference to spot_light.rs as well

https://github.com/bevyengine/bevy/blob/main/crates/bevy_render/src/maths.wgsl#L72-L87

[JeroenHoogers]

Great suggestion, thank you! I was not aware of this function's existence. Definitely looks like a cleaner solution to just call this instead.