This is part of the work specified by github Issue 586, allowing the
ability to save out the overlay in the TextureViewer. If no overlays
are on then there is no option to save the overlay. Currently there is
no option to remap the overlay to a grayscale or absolute value range
before saving. This can be a future task.
NOTE: the overlay texture resource that's saved out is not the blended
texture that the user will see in the TextureViewer, it is just the
overlay itself. The ability to save out the blended texture would be a
future task.
* This goes all the way back to the first iterations where these were
the only structures and 'Fetch' referred to them returning data from
the core code to the UI.
baldurk