Fix and clean up skeletal animation (#15722)

* Fix attachments lagging behind their parents (#14818)
* Fix animation blending (#14817)
* Bring back cool guy as another .x smoke test
* Add .x mesh loader unittest
* Do bounding box & matrix calculation at proper point in time
* Remove obsolete `SAnimatedMesh`

src/unittest/test_irr_x_mesh_loader.cpp

@@ -0,0 +1,111 @@
+// Luanti
+// SPDX-License-Identifier: LGPL-2.1-or-later
+
+#include "catch_amalgamated.hpp"
+#include "content/subgames.h"
+#include "filesys.h"
+
+#include "irrlichttypes.h"
+#include "irr_ptr.h"
+
+#include "EDriverTypes.h"
+#include "IFileSystem.h"
+#include "IReadFile.h"
+#include "ISceneManager.h"
+#include "SkinnedMesh.h"
+#include "irrlicht.h"
+
+#include "catch.h"
+#include "matrix4.h"
+
+TEST_CASE("x") {
+
+const auto gamespec = findSubgame("devtest");
+
+if (!gamespec.isValid())
+	SKIP();
+
+irr::SIrrlichtCreationParameters p;
+p.DriverType = video::EDT_NULL;
+auto *driver = irr::createDeviceEx(p);
+
+REQUIRE(driver);
+
+auto *smgr = driver->getSceneManager();
+const auto loadMesh = [&] (const io::path& filepath) {
+	irr_ptr<io::IReadFile> file(driver->getFileSystem()->createAndOpenFile(filepath));
+	REQUIRE(file);
+	return smgr->getMesh(file.get());
+};
+
+const static auto model_stem = gamespec.gamemods_path +
+		DIR_DELIM + "testentities" + DIR_DELIM + "models" + DIR_DELIM + "testentities_";
+
+SECTION("cool guy") {
+	const auto *mesh = dynamic_cast<irr::scene::SkinnedMesh*>(loadMesh(model_stem + "cool_guy.x"));
+	REQUIRE(mesh);
+	REQUIRE(mesh->getMeshBufferCount() == 1);
+
+	auto getJointId = [&](auto name) {
+		return mesh->getJointNumber(name).value();
+	};
+
+	const auto root = getJointId("Root");
+	const auto armature = getJointId("Armature");
+	const auto armature_body = getJointId("Armature_body");
+	const auto armature_arm_r = getJointId("Armature_arm_r");
+
+	std::vector<core::matrix4> matrices;
+	matrices.reserve(mesh->getJointCount());
+	for (auto *joint : mesh->getAllJoints()) {
+		if (const auto *matrix = std::get_if<core::matrix4>(&joint->transform))
+			matrices.push_back(*matrix);
+		else
+		 	matrices.push_back(std::get<core::Transform>(joint->transform).buildMatrix());
+	}
+	auto local_matrices = matrices;
+	mesh->calculateGlobalMatrices(matrices);
+
+	SECTION("joints are topologically sorted") {
+		REQUIRE(root < armature);
+		REQUIRE(armature < armature_body);
+		REQUIRE(armature_body < armature_arm_r);
+	}
+
+	SECTION("parents are correct") {
+		const auto get_parent = [&](auto id) {
+			return mesh->getAllJoints()[id]->ParentJointID;
+		};
+		REQUIRE(!get_parent(root));
+		REQUIRE(get_parent(armature).value() == root);
+		REQUIRE(get_parent(armature_body).value() == armature);
+		REQUIRE(get_parent(armature_arm_r).value() == armature_body);
+	}
+
+	SECTION("local matrices are correct") {
+		REQUIRE(local_matrices[root].equals(core::IdentityMatrix));
+		REQUIRE(local_matrices[armature].equals(core::IdentityMatrix));
+		REQUIRE(local_matrices[armature_body] == core::matrix4(
+			-1,0,0,0,
+			0,0,1,0,
+			0,1,0,0,
+			0,2.571201,0,1
+		));
+		REQUIRE(local_matrices[armature_arm_r] == core::matrix4(
+			-0.047733,0.997488,-0.05233,0,
+			0.901521,0.020464,-0.432251,0,
+			-0.430095,-0.067809,-0.900233,
+			0,-0.545315,0,1,1
+		));
+	}
+
+	SECTION("global matrices are correct") {
+		REQUIRE(matrices[armature_body] == local_matrices[armature_body]);
+		REQUIRE(matrices[armature_arm_r] ==
+			matrices[armature_body] * local_matrices[armature_arm_r]);
+	}
+}
+
+driver->closeDevice();
+driver->drop();
+}