Different mask handling when saving to PNG or GeoTIFF

[pnuu]

Code Sample, a minimal, complete, and verifiable piece of code

from satpy import Scene
import glob
fnames = glob.glob("/home/lahtinep/data/satellite/geo/*GOES15*201802201300*")
glbl = Scene(reader='hrit_goes', filenames=fnames)
glbl.load(['cloud_overview'])
glbl.save_dataset('cloud_overview', '/tmp/cloud_overview.tif')
glbl.save_dataset('cloud_overview', '/tmp/cloud_overview.png')

Problem description

The masking of missing values is different when saving to PNG or GeoTIFF. Saving to PNG handles masking correctly. With GeoTIFF all the pixels where any of the channels have masked pixels results those pixels masked also in the output image. This results in "holes" in the images also on areas where there shouldn't be any.

Versions of Python, package at hand and relevant dependencies

Tested with SatPy maint/0.8 branch, might affect also develop and master.