vulkan: remove shell call from vulkan-shaders-gen tool, revert file check

[0cc4m]

This removes the shell invocation from the vulkan-shaders-gen tool, and instead splits the string and feeds separate arguments into execvp. This way, no shell is needed. I also removed the file check on glslc.

@MaggotHATE Please check if this works for you.

[MaggotHATE]

It compiles completely with glslc passed as the argument on that portable setup, generations works, tested with Magistral 2509. Thank you!

Edit: whoops, looks like I've tested before the latest changes.

[0cc4m]

@MaggotHATE Sorry about the changes, can you test the latest version?

[MaggotHATE]

Tested again, works with the same setup. I should, however, point out two things:

1. Since I'm still using the old compilation of shaders (i.e. to a single .hpp file), I cannot use some changes in this PR, but they are minuscule. Most changes work without errors.
2. Currently, shaders compilation is fast, definitely faster than the initial solution in this PR. While I liked how elegant it looked, increased compilation time was probably not intended.

Tested again on latest commit, all good, thank you!