### 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)

ERR_FAIL_COND_V(p_peering_bit == TileSet::CELL_NEIGHBOR_MAX, -1);
	ERR_FAIL_COND_V(!is_valid_bit[p_peering_bit], -1);
	return bits[p_peering_bit];
}
void TileSet::TerrainsPattern::from_array(Array p_terrains) {
	set_terrain(p_terrains[0]);
	int in_array_index = 1;
	for (int i = 0; i < TileSet::CELL_NEIGHBOR_MAX; i++) {
		if (is_valid_bit[i]) {
			ERR_FAIL_INDEX(in_array_index, p_terrains.size());
			set_terrain_peering_bit(TileSet::CellNeighbor(i), p_terrains[in_array_index]);
			in_array_index++;
		}
	}
}
Array TileSet::TerrainsPattern::as_array() const {
	Array output;
	output.push_back(get_terrain());
	for (int i = 0; i < TileSet::CELL_NEIGHBOR_MAX; i++) {
		if (is_valid_bit[i]) {
			output.push_back(bits[i]);
		}
	}
	return output;
}
TileSet::TerrainsPattern::TerrainsPattern(const TileSet *p_tile_set, int p_terrain_set) {