Add glTF 2.0 export support

[killerdevildog]

Summary

Add the ability to export 3D models to glTF 2.0 format (JSON + separate binary/texture files), complementing the existing GLB binary export.

Motivation

glTF is the widely adopted open standard for 3D assets. While GLB (binary glTF) is already supported, the standard glTF JSON format is useful for:

• Human-readable scene inspection and debugging
• Workflows that require separate texture files (e.g., web deployment, texture swapping)
• Tools and pipelines that expect non-embedded assets

Changes

• New files: gltf_file.h / gltf_file.cc — glTF 2.0 writer modeled after the existing GlbFileWriter
• Menu integration: Added Export as glTF... action in the File menu, placed alongside the existing GLB export
• Texture handling: Base color, normal, and ORM (occlusion/roughness/metallic) textures are saved as separate PNG files alongside the .gltf and .bin files
• Build system: Added new source/header entries to application.pro

Implementation Details

• Follows the same architecture and patterns as the existing GlbFileWriter
• Full glTF 2.0 spec compliance: asset metadata, scenes, nodes, meshes, accessors, buffer views, materials, textures, and samplers
• Uses nlohmann/json (already a project dependency via json.hpp)

Testing

• Export produces valid .gltf, .bin, and texture .png files
• Verified output loads correctly in external glTF viewers