roughness is exported incorrectly, when some channles are unchecked

[soundsbeard]

ArmorPaint version: v0.9 (2022-07-27) - 43f249a

OS/device including version: non-DXR i guess all specs: https://controlc.com/a4437c7a

Issue description: exported roughness is incorrect, if some unwanted channels are unchecked.

for example: i want to have only albedo, rough and metal. i have few layers (no matter fill or paint) with variations of these 3 channels.

i uncheck all the rest of the channels on all layers

in viewport roughness looks correctly

but when i try to export, exported roughness texture is solid color #545454

the same goes with metal map

i figured out, that in order to get correct roughness on export, these channels are obligatory: even if i don't need occlusion and height - i need to check them.

Steps to reproduce: here is a project file https://sam.nl.tab.digital/s/8od8fzS9Gq36oKn