Add example with `-2147483648`

In 4.1.3 Integers section of the spec there is a set of examples listing some valid/invalid integers. Specifically it lists:

```
2147483648 // OK. Evaluates to -2147483648 (the literal set the sign bit)
```

But, if you do `-2147483648`, then you also get `-2147483648` (the negation of -2147483648 is -2147483648), you can also do `-(-2147483648)` to get the same value.

So, it might be worthwhile noting that the negation of `-2147483648` is itself?

(This is different then say `-2147483649` which becomes `2147483647` because `2147483649` is `-2147483647` which then negates as expected)

Start with section 4.1.3, Integers, of the GLSL specification and review the existing valid and invalid integer examples. Update the example text to note the behavior of negating -2147483648, then verify that the surrounding integer semantics remain consistent.

Written by the indexing model from the issue text.