MRG, FIX, VIZ: correctly triage extrapolation mask for topos based on channel type

[drammock]

failure to pass through ch_type was the culprit; the underlying plotting function thought everything was 'eeg'.

[drammock]

Here's the rendering showing the correct mask:
https://23871-1301584-gh.circle-artifacts.com/0/dev/auto_tutorials/epochs/plot_20_visualize_epochs.html#plotting-the-power-spectrum-of-epochs

[larsoner]

Thanks @drammock

[agramfort]

this could be backported

…

[larsoner]

I think we're going to release in a couple of weeks, I don't see much point