Streaplots with multicoloured lines bug

Streamplots with multicoloured lines arent properly coloured when the tikzplot is created.
I solved the issue by adding the following snippet to `_line2d.draw_linecollection`  (has to be before ` edgecolors = obj.get_edgecolors()`):

```
obj.update_scalarmappable()
```

I'm not sure if it solves all cases or there are similar issues elsewhere, but this worked for me!