### 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 separation;
}
void TileSetAtlasSource::set_texture_region_size(Vector2i p_tile_size) {
	if (p_tile_size.x <= 0 || p_tile_size.y <= 0) {
		WARN_PRINT("Atlas source tile_size should be strictly positive.");
		texture_region_size = Vector2i(MAX(1, p_tile_size.x), MAX(1, p_tile_size.y));
	} else {
		texture_region_size = p_tile_size;
	}
	_queue_update_padded_texture();
	emit_changed();
}
Vector2i TileSetAtlasSource::get_texture_region_size() const {
	return texture_region_size;
}
void TileSetAtlasSource::set_use_texture_padding(bool p_use_padding) {
	if (use_texture_padding == p_use_padding) {
		return;
	}
	use_texture_padding = p_use_padding;
	_queue_update_padded_texture();
	emit_changed();
}
bool TileSetAtlasSource::get_use_texture_padding() const {
	return use_texture_padding;
}
Vector2i TileSetAtlasSource::get_atlas_grid_size() const {