Clarify GLSL for-loop scoping rules about glsl

[johnstiles-google]

The docs do say "Variables declared in init-expression or condition-expression are only in scope until the end of the sub-statement of the for loop." but don't indicate that the for-statement itself actually introduces a new scope, only that the sub-statement does not introduce one.

Interestingly, the grammar includes statement_no_new_scope to cover this case, but doesn't clearly indicate where scopes are introduced.

[johnstiles-google]

I also discovered Section 4.2 states: "If (a variable) is declared in a while test or a for statement, then it is scoped to the end of the following sub-statement." This is a great clue but it doesn't actually clarify that for-statements are scopes.

In particular, we can demonstrate that in glslang, for-statements are scopes even when they do not declare any variables at all:

void main() {
    int x=1; for (;;) int x=2;
}

This compiles in glslang.