Part of #3321. This covers the five classes still unticked in its checklist: AxisAlignedBox, Matrix3, Matrix4, Affine3 and Quaternion.

The constructors used to define ZERO, IDENTITY, CLIPSPACE2DTOIMAGESPACE, BOX_NULL and BOX_INFINITE are now constexpr. The definitions in the .cpp files are unchanged, but they now meet the C++14 rule for constant initialization: a static object initialized by a constexpr constructor with constant arguments. They are therefore set before any dynamic initialization runs, so another translation unit (or a static library user) that reads them from its own static initializer no longer depends on initialization order.

This differs from #3313/#3315: the constants stay objects (static const Matrix4 IDENTITY;) instead of becoming references, so the ABI doesn't change. If you'd rather keep the #3315 pattern for consistency, that is a small follow-up on top of the constexpr constructors.

Changes:

• Matrix3(9 reals), Quaternion(w, x, y, z) and both AxisAlignedBox constructors initialize their members in the member initializer list and are constexpr. The AxisAlignedBox values are the same as before: (-0.5) to (0.5), with the given extent.
• Matrix4(16 reals) and Affine3(12 reals) are constexpr. C++14 requires a constexpr constructor to initialize every member, and m lives in TransformBase. So this adds a protected constexpr TransformBase/TransformBaseReal constructor that takes all 16 entries, and the two constructors forward to it. Affine3 passes 0, 0, 0, 1 for the last row, as before.
• Matrix3::EPSILON, msSvdMaxIterations and Quaternion::msEpsilon are scalars with constant initializers, so they were already fine.

Tests (Tests/OgreMain/src/General.cpp):

• constexpr variables built with each of these constructors at namespace scope. This compiles only if the constructors are usable in constant expressions, which is what the constants rely on.
• Math.ConstexprConstructors checks the entry order (row-major, Affine3's last row) and the two box extents.
• Math.StaticConstants checks the values of all eleven constants.

Verification (macOS arm64, Apple clang, Unix Makefiles generator, Debug, OGRE_BUILD_LIBS_AS_FRAMEWORKS=OFF, all render systems off, samples and tests on):

• Without the header changes, General.cpp fails to compile: constexpr variable cannot have non-literal type for Matrix3, Matrix4, Affine3, Quaternion and both AxisAlignedBox variables (6 errors).
• Dynamic initializers (___cxx_global_var_init* in nm -m of each object file), before -> after: OgreMatrix3.cpp.o 3 -> 1, OgreMatrix4.cpp.o 6 -> 1, OgreQuaternion.cpp.o 3 -> 1, OgreAxisAlignedBox.cpp.o 3 -> 1. The remaining one is the same std::string initializer that OgreVector.cpp.o (already converted) also has.
• Test_Ogre --gtest_filter=-UseCustomCapabilitiesTests*:
  • on master: 159 tests, 154 passed, 2 skipped (DotSceneTests.exportImport, QuaternionTests.Precision), 3 failed;
  • with the change: 161 tests, 156 passed, the same 2 skipped and the same 3 failed.
  • The 3 failures are Root.shutdown, MaterialSerializer.Basic and Image.Compressed. All three come from this non-framework build: they can't dlopen Contents/Frameworks/Plugin_ParticleFX.framework.
• make of all targets (including the tools and SampleBrowser) succeeds with the change.

Not run: GCC, MSVC, Linux/Windows builds, the SWIG bindings, visual tests, and an Xcode-generator build.