Supporting tail-recursion in GLSL

GLSL currently does not support tail recursion, but this feature could be easily implemented using tail-call optimization. For example, this function is not yet compatible with GLSL:

```
int fib(int n)
{
if (n <= 1)
return n;
return fib(n - 1) + fib(n - 2);
}
```

But it's possible to eliminate recursion here, replacing the recursive call with a for-loop:

```
int fib(int n)
{
int a = 0, b = 1, c, i;
if (n == 0)
return a;
for (i = 2; i <= n; i++)
{
c = a + b;
a = b;
b = c;
}
return b;
}
```
Are there any plans to implement tail-recursion in a future version of GLSL?

Start by reviewing the GLSL specification and the existing recursion or function-call rules; the issue names no specific file or test. Determine whether tail recursion belongs in the language specification and define the required behavior and validation before implementation.

Written by the indexing model from the issue text.