Fix oslc missing a write error on function parameter

[lgritz]

Fixes #1416

In OSL, function parameters must be marked as output or they are not
supposed to be writable. In issuing errors when a write is performed on
something not writable, we missed an interesting case:

void g(output float b) {
    b = 1;  // ok, since b is marked as output
}

void f(float a) {
    g(a);  // <--- should be disallowed, g is writing to f's non-output param
}

float x;
f(x);

This patch catches cases like these. The new error looks like this:

test.osl:22: error: cannot write to non-output parameter "a", as passed to function g() as arg 1 (output parameter "b")

Signed-off-by: Larry Gritz [email protected]

[lgritz]

@lkerley or others, does this seem ok? Is the form of that error message helpful?

[lgritz]

Lee said LGTH