### 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(p_terrain < -1);
	terrain = p_terrain;
}
int TileSet::TerrainsPattern::get_terrain() const {
	return terrain;
}
void TileSet::TerrainsPattern::set_terrain_peering_bit(TileSet::CellNeighbor p_peering_bit, int p_terrain) {
	ERR_FAIL_COND(p_peering_bit == TileSet::CELL_NEIGHBOR_MAX);
	ERR_FAIL_COND(!is_valid_bit[p_peering_bit]);
	ERR_FAIL_COND(p_terrain < -1);
	
	if (p_terrain >= 0 && bits[p_peering_bit] < 0) {
		not_empty_terrains_count++;
	} else if (p_terrain < 0 && bits[p_peering_bit] >= 0) {
		not_empty_terrains_count--;
	}
	bits[p_peering_bit] = p_terrain;
}
int TileSet::TerrainsPattern::get_terrain_peering_bit(TileSet::CellNeighbor p_peering_bit) const {