Add macOS support

[diminox-kullwinder]

Summary

This PR adds native macOS support to stl-thumb, enabling headless thumbnail rendering on Apple systems.

Changes

1. macOS headless display (src/lib.rs)

Added create_headless_display implementation for target_os = "macos" using the same pattern as Windows (simpler than Linux's multi-fallback approach).

2. GLSL shader updates (all shaders)

macOS's OpenGL core profile requires GLSL 150+. Updated all shaders:

• #version 120 → #version 150
• attribute → in
• varying → out / in
• texture2D() → texture()
• gl_FragColor → explicit out vec4 fragColor

These changes are backwards compatible with Linux/Windows as GLSL 150 is widely supported.

Testing

Tested on macOS 14.x (Sonoma) with Intel x86_64:

• STL files: ~0.6s render time
• OBJ files: ~0.3s render time
• 3MF files: ~20s render time (complex format)

Notes

• macOS deprecated OpenGL in favor of Metal, but OpenGL still works and is the path of least resistance for cross-platform support
• The headless context approach is simpler on macOS than Linux (no surfaceless/osmesa fallbacks needed)