Add normal maps

[kzsh]

Add normal maps

Note

this builds on #2603, which laid most of the work out already.

Wires normal mapping through the mesh pipeline, ships normal maps for the
models in the resource packs, and fixes the tangent basis so mirrored UV
islands are lit correctly.

Changes

Tangent basis (src/mesh.h, src/mesh.cpp, resources/shaders/objectShader.shader)

The mesh loader derived a tangent from the UV parameterisation but discarded
the sign of the determinant, and the shader assumed every UV island was
right-handed. Models that mirror their UVs across the hull centreline (most of
the freighters and destroyers) rendered one half with an inverted bitangent,
producing a hard seam with the two sides lit from opposite directions.

MeshVertex::tangent becomes a vec4, with the handedness in w, and the
shader rebuilds the bitangent as cross(n, t.xyz) * t.w. The 1/determinant
scale is dropped, since the shader normalizes the basis, which also removes a
division by zero on triangles with degenerate UVs.

Replication (src/multiplayer/rendering.cpp)

normal_texture.name was never replicated, so clients never received the map.

Texture unit reset (src/systems/rendering.cpp)

The active texture unit is now also reset when a mesh has a normal map but no
specular or illumination map. Every model in model_data.lua that sets a
normal map also sets a specular map, so no shipped asset currently hits this,
but the loader no longer depends on that.

Model viewer (src/screenComponents/rotatingModelView.cpp)

Adds the normal map toggle to the debug view, so the map's contribution can be
judged against the flat-shaded model.

Assets (packs/, scripts/model_data.lua)

46 setNormalMap() entries. space_tug and space_cruiser_4 are not updated;
they need normal maps authored from scratch.

Note for the release: stale msgamedev.pack

msgamedev.pack is split into msgamedev-1.pack and msgamedev-2.pack, because
the added textures push the combined pack past GitHub's 100 MB file limit.

The rename means an in-place upgrade that unpacks over an existing install
may accidentally load the old msgamedev.pack, missing out on the new texture
data.

If that is a concern, the split can keep the name msgamedev.pack for one
half and add only msgamedev-2.pack, so the old file is always overwritten.
Happy to change it either way.