* [glsl-in] use TypeInner::scalar_kind for casts

* [glsl-in] Use .ok_or for casts

Otherwise the backends will panic on e.g. `vec4 local = Global;` if the global
isn't used anywere else in the function.

This pretty trivial change unfortunately required a loop to mutably borrow both
builder and manager at the same time, so I had to separate them. Perhaps a
cleaner solution would be to use an object with internal state instead of free
functions.

while, do-while and for loops

i.e. if-else and switch-case

* msl-out: Use io::Write instead of fmt::Write

* msl-out: merge std use

Header blocks were incorrectly overwritten.
format

* [glsl-in] Add initial pre-processor

* [glsl-in] Tweak preprocessor after review

- Add IfState struct
- Use FastHashMap to pre-defines
- Use ok-or() for some errors

* [glsl-in] Remove Option in defines arg