Update xatlas to upstream 5571fc7, fixes #44017

[Wavesonics]

Fixes a crash where the cross product's length is smaller than the epsilon value.

Fixes #44017.

[Wavesonics]

@Calinou I can change this to a .patch, but I'm not sure if this is the actual the answer. If anyone else has any knowledge of whats going on here I'd love some insight.

In the 3.x branch this fix works, the output can be opened in Godot and it looks fine. But in 4.0 it looks broken, at least the textures aren't loading correctly. Which I don't know if that is related to the original bug or not.

[Wavesonics]

Ah okay, I think the real fix here is to take the latest from xatlas.

The normalize() function has been rewritten, now instead of using epsilon to check for non-zero, it instead simply checks that the length is non-negative. I tested this PR and it fixed my GLB import crash.

static Vector3 normalize(const Vector3 &v)
{
	const float l = length(v);
	XA_DEBUG_ASSERT(l > 0.0f); // Never negative.
	const Vector3 n = v * (1.0f / l);
	XA_DEBUG_ASSERT(isNormalized(n));
	return n;
}

instead of:

static Vector2 normalize(const Vector2 &v, float epsilon)
{
	float l = length(v);
	XA_DEBUG_ASSERT(!isZero(l, epsilon));
	XA_UNUSED(epsilon);
	Vector2 n = v * (1.0f / l);
	XA_DEBUG_ASSERT(isNormalized(n));
	return n;
}

This PR updates xatlas to 5571fc7ef0d06832947c0a935ccdcf083f7a9264, which is the current head of their master.

[fire]

There's a reasonable assumption that we don't have to go through a regression test suite to test if things broke for xatlas, but we probably should.. Would like to merge for wider testing.

[Wavesonics]

@fire if you wouldn't mind, there is one line of the change that I had to make that I didn't fully understand. I started a review of it. The function signature changed, and the ParameterizeOptions type was removed from xatlas. So I took a crack at getting it to compile just test my theory. But I don't totally understand what it was supposed to be doing on this line.

[Wavesonics]

Someone who knows more than me, I don't fully understand this change. It made it build, but someone should check my work?

[fire]

Instead of using:

	pack_options.maxChartSize = 4096;
	pack_options.blockAlign = true;
	pack_options.padding = 1;
	pack_options.texelsPerUnit = 1.0 / p_texel_size;

You set it to the default. This is arguably a bug.

[fire]

You probably want xatlas::Generate(atlas, chart_options, pack_options);

[akien-mga]

Could you amend the commit log to be more explicit about the change? It's true that your intent is to fix #44017, but what the commit does first and foremost is updating xatlas (which changes a lot of things, including a change that happens to fix #44017).

So a better commit log would be:

xatlas: Sync with upstream 5571fc7

Fixes #44017 by changing the `normalize()` function.

Please also update the commit hash for xatlas in thirdparty/README.md.

[Wavesonics]

@akien-mga updated the commit log and the README

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.2.4.