Enable asynchronous shader compilation

[syntheticmagus]

Per comments by @CedricGuillemet and @bghgary on #370, the potential for increasingly complex work required to traverse and transpile shaders under certain circumstances will likely justify moving shader transpilation to a thread pool to be done asynchronously. This work should be a relatively simple matter of combining patterns already established for mipping with the shader transpilation pipeline, but some adaptation may be required to ensure rendering calls are resilient to potential delays in the completion of asynchronous compilations.