Channel Remap: Information not preserved when remapping to a non-existent destination channel

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Tested versions

• Reproducible in: v4.5.dev.mono.gh [428a762]

System information

Godot v4.5.dev.mono (428a762) - Linux Mint 22.1 (Xia) on X11 - X11 display driver, Multi-window, 1 monitor - Vulkan (Forward+) - dedicated NVIDIA GeForce GTX 960 (nvidia; 550.144.03) - AMD FX(tm)-6300 Six-Core Processor (6 threads)

Issue description

Recently, the feature Channel Remap was added, but it has a behavior that can be considered problematic when trying to remap a channel to a non-existent channel in the texture.

As you can see, Unity allowed me to move the red channel to the alpha channel.

In the case of Godot, it does create the alpha channel but leaves it blank.

Steps to reproduce

1. Select an RGB texture.
2. Go to the import dock.
3. Remap the Red channel to Zero.
4. Remap the Blue channel to Zero.
5. Remap the Alpha channel to Red.
6. Reimport the texture.

Minimal reproduction project (MRP)

Here the textures.