### 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)

Vector<int> data;
	data.resize(pattern.size() * 3);
	int *w = data.ptrw();
	
	int idx = 0;
	for (const KeyValue<Vector2i, TileMapCell> &E : pattern) {
		uint8_t *ptr = (uint8_t *)&w[idx];
		encode_uint16((int16_t)(E.key.x), &ptr[0]);
		encode_uint16((int16_t)(E.key.y), &ptr[2]);
		encode_uint16(E.value.source_id, &ptr[4]);
		encode_uint16(E.value.coord_x, &ptr[6]);
		encode_uint16(E.value.coord_y, &ptr[8]);
		encode_uint16(E.value.alternative_tile, &ptr[10]);
		idx += 3;
	}
	return data;
}
void TileMapPattern::set_cell(const Vector2i &p_coords, int p_source_id, const Vector2i p_atlas_coords, int p_alternative_tile) {