extract_unity_headers: Save json index of CRCs to headers
shadertool.py: Add --lookup-header-json and --in-place options

--in-place will re-write the modified shader in place.

--lookup-header-json will lookup the shader in the index created by
export_unity_shaders and add the header. If no other installation
options are specified this implies --in-place --no-convert.
shadertool: Reduce sleep on parsing failure