mix() BIF is broken for differently transparent colors

output alpha was calculated based on color1 only - leading to no alpha-channel mixing at all.

Example of old broken behavior:

```stylus
body
    foo: mix( #000,  rgba(#000,0.6) );
    bar: mix( rgba(#000,0.6),  #000 );
    baz: mix( rgba(#000, 0.6), #000, 25% );
```

incorrectly outputs:

```css
body {
    foo: #000;
    bar: rgba(0,0,0, 0.6);
    baz: rgba(0,0,0, 0.6);
}
```

...  
But should be (and is with this PR):

```css
body {
    foo: rgba(0,0,0, 0.8);
    bar: rgba(0,0,0, 0.8);
    baz: rgba(0,0,0, 0.9);
}
```