Support for calling into c functions from compose scripts

- Interim checkin, still working on these features but all builds and a
  lot is working
- system_api.h/cpp holds functions callable from compose scripts
- system_api_meta.cpp defines metadata about parameters so compiler can
  check params to these functions.
- system_api_meta.cpp will be auto-generated, but is currently a manual
  process
- The auto-generatoin will happen by parsing system_api.h and building
  the api metadata structs dynamically
- Also refined some stuff around models, meshes, materials and shapes.
- Lots of code written months ago is finally ready to be consumed, but
  there are some issues to work through
- Current target is a compose script that creates a box model and sends
  it to the renderer for display