Moved the global writing part to it's own function
Merged the texture and the global loop
Applied some suggestions from kvark
Added a topological sorting of the call graph before writing
Removed the struct block loop and now every struct is written
Organized the main backend file a bit

The backend now works in a similar way to the msl backend
Should require less loops, allocations and backtracking
Overall just better to work with it

Added a build script which reads a file with the glsl keywords and
generates a slice to be used in the Namer

Added a way to build a call graph

* [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.