Add support for trace opacity in toself fills

[etpinard]

resolves #884

As mentioned in #884, trace opacity for scatter trace with non-'none' fills are not honoured in the drawing code. This decision was made back in 2015 to (in brief) make sure the opacity of tonext* trace couplets do not become inconsistent.

But, that old fix for the above edge cases do not apply to toself fills which was added in 1.10.0.

This PR makes trace opacity be honoured for toself fills.

[mdtusz]

lgtm! 💃

[tsando]

Is there any way to set the opacity for tonexty? This example only seems to cover toself and I'm still having the bug if using tonexty. Only way to set opacity is to use toself. Thanks

[etpinard]

@tsando

Is there any way to set the opacity for tonexty?

No. We explicitly unset it here as it can lead to unwanted effects. The behavior can be seen here.

To adjust tonext* fill transparencies, use the alpha channel the fillcolor e.g. 'rgba(255,0,0,0.2)'.