Changes in this release:
- Support GLSL_EXT_shader_tile_image
- Support GL_EXT_ray_tracing_position_fetch
- Support custom include callbacks via the C API
- Add preamble-text command-line option
- Accept variables as parameters of spirv_decorate_id
- Fix generation of conditionals with a struct result
- Fix double expansion of macros
- Fix DebugCompilationUnit scope
- Improve line information