BMP image loader fails to import images with 8, 4 and 1 bit depths

[Xrayez]

Godot version:
3.2

OS/device including version:
Windows 10

Issue description:
Try to load .bmp images with 8, 4 and 1 bit depth, it will fail to import them.
Loading 24 bit images works alright since #20038.

I know the bmp hate but here it is 🐛.

Minimal reproduction project:
bmp-8bit-import.zip

[akien-mga]

CC @SaracenOne

[Xrayez]

@SaracenOne note that I'm currently working on palette support in images that could help to solve this. I've made some progress already on this, but you could fix it independently of whether the feature is merged or not (#28013).

[Xrayez]

For reference, the issue with importing 8-bit bmp images was fixed as part of my PR using the new palette API 45917ac, I suppose it would be relatively easy to do the same with 4-bit and 1-bit images done.