### Godot version

5ee983188de97ae027f9b9c1443438063f708a7e

### System information

Windows 11

### Issue description

Godot editor will throw an error and fail to generate preview icons for terrain tiles which use a base image which is not RGBA8 format.
`core\io\image.cpp:2788 - Condition "format != p_src->format" is true.`

### Steps to reproduce

- Import a non-RGBA8 image into Godot.
- Create a TileMap
- Create an associated TileSet
- Create a terrain for this tileset
- Add the image as an atlas
- Go to the paint tab
- Paint the terrain on the TileSet
- An error will occur any time terrains are edited in the TileSet editor or selected in the TileMap editor terrain tab.

### Minimal reproduction project

[tileset_terrain_bug.zip](https://github.com/godotengine/godot/files/13264213/tileset_terrain_bug.zip)

return texture;
}
void TileSetAtlasSource::set_margins(Vector2i p_margins) {
	if (p_margins.x < 0 || p_margins.y < 0) {
		WARN_PRINT("Atlas source margins should be positive.");
		margins = Vector2i(MAX(0, p_margins.x), MAX(0, p_margins.y));
	} else {
		margins = p_margins;
	}
	_queue_update_padded_texture();
	emit_changed();
}
Vector2i TileSetAtlasSource::get_margins() const {
	return margins;
}
void TileSetAtlasSource::set_separation(Vector2i p_separation) {
	if (p_separation.x < 0 || p_separation.y < 0) {
		WARN_PRINT("Atlas source separation should be positive.");
		separation = Vector2i(MAX(0, p_separation.x), MAX(0, p_separation.y));
	} else {
		separation = p_separation;
	}
	_queue_update_padded_texture();
	emit_changed();
}
Vector2i TileSetAtlasSource::get_separation() const {