Use sized types in SPIR-V generation

The SPIR-V generator currently uses `unsigned int` for all its internal representation. However there is no guarantee for this type to be 32 bits (one SPIR-V word) wide.

There are two solutions to this:
- Change the official Khronos headers to generate `uint32_t` for the `Id` typedef and underlying types for enums.
- Change the glslang code to only use `uint32_t` internally.

I would prefer the first option (together with other improvements to the C++ headers) but that requires changing the Khronos code generator and I have no idea whether that is public and if so where to find it/contribute to it (the Khronos bugtracker only has a section for the specification, not the interface files).

In any case, the glslang code as it is right now may produce faulty SPIR-V binaries or fail to load valid SPIR-V binaries if the condition `sizeof(unsigned int) * CHAR_BIT == 32` does not hold on the host platform. As a plus, if the platform does not have an exact 32 bit wide integer the code would fail to compile as the exactly sized typedefs are optional.

There may be similar issues with `float` and `double` if they don't follow the IEEE 754 standard or have unexpected sizes.

The issue names no files, tests, or entry points. Start by locating the SPIR-V generator and the Khronos-generated headers, then trace where unsigned int, enum underlying types, float, and double are used. Done should mean the generator’s host-size assumptions are addressed and portability is covered by relevant build or validation checks.

Written by the indexing model from the issue text.