### 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_set < 0);
	for (int i = 0; i < TileSet::CELL_NEIGHBOR_MAX; i++) {
		is_valid_bit[i] = (p_tile_set->is_valid_terrain_peering_bit(p_terrain_set, TileSet::CellNeighbor(i)));
		bits[i] = -1;
	}
	valid = true;
}
const int TileSet::INVALID_SOURCE = -1;
const char *TileSet::CELL_NEIGHBOR_ENUM_TO_TEXT[] = {
	PNAME("right_side"),
	PNAME("right_corner"),
	PNAME("bottom_right_side"),
	PNAME("bottom_right_corner"),
	PNAME("bottom_side"),
	PNAME("bottom_corner"),
	PNAME("bottom_left_side"),
	PNAME("bottom_left_corner"),
	PNAME("left_side"),
	PNAME("left_corner"),
	PNAME("top_left_side"),
	PNAME("top_left_corner"),
	PNAME("top_side"),
	PNAME("top_corner"),
	PNAME("top_right_side"),
	PNAME("top_right_corner"),
};
void TileSet::set_tile_shape(TileSet::TileShape p_shape) {