### Transparency option in streamline plot

In most graphics functions it's possible to set the transparency of the plot with the `alpha`

option. In a streamline plot, e.g. take

```
sage: var('x, y')
sage: streamline_plot((y, -x**2 * y - x + y), (x, -3, 3), (y, -3, 3), density=1.4, color='black')
```

it doesn't work:

```
sage: var('x, y')
sage: streamline_plot((y, -x**2 * y - x + y), (x, -3, 3), (y, -3, 3), density=1.4, color='black', alpha=0.5)
...
WARNING: Ignoring option 'alpha'=0.500000000000000
...
```

Is there a workaround for this?