Don't warn about unused globals when both defined implicitly and through std

[NutchapolSal]

here's an example:

a game has the function foo it calls, but it lets the lua file define it

function foo()
    local x = math.random(1, 999)
    draw.bars_on_screen(x)

    x = math.some_stuff(10, x)
    draw.some_more_baz(x)
end

i have allow_defined enabled, and draw and it's fields defined in std

foo() doesn't come up in the rest of the lua file, so it now warns that foo() is an unused global

i'm suggesting that when this new option is enabled then if it is both defined in std and implicitly inside the file, then don't warn about it.