Fix entrypoint naming in glsl backend.

[csyonghe]

When there is

[shader("compute")]
void inner() { ... }

[shader("compute")]
void outer() { inner(); }

And the user trys to compile outer to glsl, we are currently generating two functions named main in the output glsl because both functions are marked as entrypoint, and there is the logic to emit all entrypoints as main. This is wrong if a user-declared entrypoint isn't being selected as the entrypoint in the current compilation session.

[jkwak-work]

Looks good to me