### 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_MSG(!allow_transform && p_flip_h, "Transform is only allowed for alternative tiles (with its alternative_id != 0)");
	flip_h = p_flip_h;
	emit_signal(SNAME("changed"));
}
bool TileData::get_flip_h() const {
	return flip_h;
}
void TileData::set_flip_v(bool p_flip_v) {
	ERR_FAIL_COND_MSG(!allow_transform && p_flip_v, "Transform is only allowed for alternative tiles (with its alternative_id != 0)");
	flip_v = p_flip_v;
	emit_signal(SNAME("changed"));
}
bool TileData::get_flip_v() const {
	return flip_v;
}
void TileData::set_transpose(bool p_transpose) {
	ERR_FAIL_COND_MSG(!allow_transform && p_transpose, "Transform is only allowed for alternative tiles (with its alternative_id != 0)");
	transpose = p_transpose;
	emit_signal(SNAME("changed"));
}
bool TileData::get_transpose() const {