Fix an issue where we could take an FBO off a freelist but not properly clean fbo_for...

[movit] / alpha_division_effect.frag

// Note: Division by zero will give inf or nan, whose conversion to
// integer types is implementation-defined. However, anything is fine for
// alpha=0, since that's undefined anyway.
vec4 FUNCNAME(vec2 tc) {
        vec4 x = INPUT(tc);
        x.rgb /= x.aaa; 
        return x;
}