Default color causing issue when missing opacity texture

[lfl-eholthouser]

When a texture cannot be found the result for that input is the default value which is currently 0. This causes issues for Standard Surface materials when the opacity texture map is missing. The (0, 0, 0) opacity value causes the material to become transparent. This is seen both when viewing the material in the Viewer and baking out the textures.

[kwokcb]

I think when the work to add support for default value was added there was some discussion about providing different defaults depending on where it was mapped to. So opacity should be 1,1,1, normalmap should be .5,.5,1 etc.

It's been a while so forget the details. Think it may be in a Slack thread or the PR / issue for this work.