### 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)

Array from;
	from.push_back(p_source_from);
	from.push_back(p_coords_from);
	ERR_FAIL_COND(!coords_level_proxies.has(from));
	coords_level_proxies.erase(from);
	emit_changed();
}
void TileSet::set_alternative_level_tile_proxy(int p_source_from, Vector2i p_coords_from, int p_alternative_from, int p_source_to, Vector2i p_coords_to, int p_alternative_to) {
	ERR_FAIL_COND(p_source_from == TileSet::INVALID_SOURCE || p_source_to == TileSet::INVALID_SOURCE);
	ERR_FAIL_COND(p_coords_from == TileSetSource::INVALID_ATLAS_COORDS || p_coords_to == TileSetSource::INVALID_ATLAS_COORDS);
	Array from;
	from.push_back(p_source_from);
	from.push_back(p_coords_from);
	from.push_back(p_alternative_from);
	Array to;
	to.push_back(p_source_to);
	to.push_back(p_coords_to);
	to.push_back(p_alternative_to);
	alternative_level_proxies[from] = to;
	emit_changed();
}
Array TileSet::get_alternative_level_tile_proxy(int p_source_from, Vector2i p_coords_from, int p_alternative_from) {
	Array from;
	from.push_back(p_source_from);
	from.push_back(p_coords_from);
	from.push_back(p_alternative_from);
	ERR_FAIL_COND_V(!alternative_level_proxies.has(from), Array());
	return alternative_level_proxies[from];
}
bool TileSet::has_alternative_level_tile_proxy(int p_source_from, Vector2i p_coords_from, int p_alternative_from) {