regionprops multichannel measurements don't work for extra_properties

[VolkerH]

Description

regionprops measurments have been supporting multi-channel intensity images as of recently.
However, when combined with extra_properties-functions that also rely on intensity the extra_properties only appear to be dispatched for one channel (at least the result is not multi-channel).

Way to reproduce

Call regionprops with a multi-channel intensity_image and an extra_properties-function that relies on intensity.
Observe the result.
(will fill in more detail if needed, a bit in a rush atm).

Version information

3.8.5 (default, Sep  4 2020, 07:30:14) 
[GCC 7.3.0]
Linux-5.8.0-48-generic-x86_64-with-glibc2.10
scikit-image version: 0.18.1
numpy version: 1.18.1