Image: Fix detecting color channels

[BlueCube3310]

Follow-up to #106437

The code for detecting the resulting channel configuration operated under the assumption that each color channel that was part of said configuration had to be present in an image.
This is the most common scenario, but the channel remapping feature made the issue more likely to come across.

Now, the configurations are selected based on the presence of the outermost channel. (i.e. A -> RGBA, B -> RGB, G -> RG, R-> R).

[fire]

I think this is a good change, but I can't review it in depth now. Consider this approval.

[Repiteo]

Thanks!