### 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 terrain_set;
}
void TileData::set_terrain(int p_terrain) {
	ERR_FAIL_COND(terrain_set < 0);
	ERR_FAIL_COND(p_terrain < -1);
	if (tile_set) {
		ERR_FAIL_COND(p_terrain >= tile_set->get_terrains_count(terrain_set));
	}
	terrain = p_terrain;
	emit_signal(SNAME("changed"));
}
int TileData::get_terrain() const {
	return terrain;
}
void TileData::set_terrain_peering_bit(TileSet::CellNeighbor p_peering_bit, int p_terrain_index) {
	ERR_FAIL_INDEX(p_peering_bit, TileSet::CellNeighbor::CELL_NEIGHBOR_MAX);
	ERR_FAIL_COND(terrain_set < 0);
	ERR_FAIL_COND(p_terrain_index < -1);
	if (tile_set) {
		ERR_FAIL_COND(p_terrain_index >= tile_set->get_terrains_count(terrain_set));
		ERR_FAIL_COND(!is_valid_terrain_peering_bit(p_peering_bit));
	}
	terrain_peering_bits[p_peering_bit] = p_terrain_index;
	emit_signal(SNAME("changed"));
}
int TileData::get_terrain_peering_bit(TileSet::CellNeighbor p_peering_bit) const {