Some LevelRenderer functions that I decompiled by hand.

LevelRenderer.h

void LevelRenderer::_buildSkyMesh()
{
	Tessellator& tessellator = Tessellator::instance;
	tessellator.begin(mce::PrimitiveMode::MODE_2, 12);
	tessellator.color(Color::BLACK);
	tessellator.vertex(0.0F, 128.0F, 0.0F);

	double x = 1.0;
	double z = 0.0;
	double f = Math::TAU * 0.1;

	for(int i = 0; i < 11; ++i)
	{
		tessellator.color(Color::WHITE);
		tessellator.vertex(x * 2000.0F, 128.0F, -(z * 2000.0F));

		x = cos(-(i * f));
		z = sin(-(i * f));
	}

	tessellator.beginIndices(0);

	for(int i = 0; i < 11; ++i)
	{
		unsigned int j = i * 0x402e161e;
		j = (i >> 31) >> (j >> 2);
		j += (j << 2);
		tessellator.triangle(0, i, (i - (j << 1)) + 1);
	}

	this->field_16E0.Mesh_4 = tessellator.end(NULL, false);
}

void LevelRenderer::renderSky(Entity& cameraEntity, float partialTicks)
{
	mce::RenderContextImmediate::get().setDepthRange(0.0F, 1.0F);

	MatrixStack::Ref projection = MatrixStack::Projection.push();

	float fov = tanf(this->getFov(partialTicks, true) * 0.0087266F);
	float asp = fov * this->_getProjectionAspectRatio();
	float f1 = this->field_1620.field_20 * 5120.0F;

	Matrix matrix;
	matrix.f84 = 1.0F;
	matrix.f96 = (f1 * 4.0F) / (2.0F - f1);
	matrix.f60 = 1.0F / fov;
	matrix.f80 = (f1 + 2.0F) / (2.0F - f1);
	matrix.f40 = 1.0F / asp;

	projection = matrix;

	f1 = this->field_1620.field_20 * 1.28F;

	MatrixStack::Ref view = MatrixStack::View.push();
	view.stack.field_C = 1;
	view.matrix.scale({f1, f1, f1});

	Color skyColor = this->dimension.getSkyColor(cameraEntity, partialTicks);
	float ambientBrightness = this->getAmbientBrightness();
	float darkness = this->_skyDarkeningFactor();

	float r = (skyColor.r * ambientBrightness) - darkness;
	float g = (skyColor.g * ambientBrightness) - darkness;
	float b = (skyColor.b * ambientBrightness) - darkness;
	float a = 1.0F;
	currentShaderColor.setColor({r, g, b, a});

	this->skyMesh.render(this->skyMaterial, 0, 0);

	this->_renderSunOrMoon(partialTicks, true);
	this->_renderSunOrMoon(partialTicks, false);
	this->_renderStars(partialTicks);

	mce::RenderContextImmediate::get().setDepthRange(0.0F, 0.7F);
}

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