### 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(!pattern.has(p_coords), TileSet::INVALID_SOURCE);
	return pattern[p_coords].source_id;
}
Vector2i TileMapPattern::get_cell_atlas_coords(const Vector2i &p_coords) const {
	ERR_FAIL_COND_V(!pattern.has(p_coords), TileSetSource::INVALID_ATLAS_COORDS);
	return pattern[p_coords].get_atlas_coords();
}
int TileMapPattern::get_cell_alternative_tile(const Vector2i &p_coords) const {
	ERR_FAIL_COND_V(!pattern.has(p_coords), TileSetSource::INVALID_TILE_ALTERNATIVE);
	return pattern[p_coords].alternative_tile;
}
TypedArray<Vector2i> TileMapPattern::get_used_cells() const {
	
	TypedArray<Vector2i> a;
	a.resize(pattern.size());
	int i = 0;
	for (const KeyValue<Vector2i, TileMapCell> &E : pattern) {
		Vector2i p(E.key.x, E.key.y);
		a[i++] = p;
	}
	return a;
}
Size2i TileMapPattern::get_size() const {
	return size;
}
void TileMapPattern::set_size(const Size2i &p_size) {