### 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_INDEX(p_terrain_set, terrain_sets.size());
	ERR_FAIL_INDEX(p_terrain_index, terrain_sets[p_terrain_set].terrains.size());
	terrain_sets.write[p_terrain_set].terrains.write[p_terrain_index].name = p_name;
	emit_changed();
}
String TileSet::get_terrain_name(int p_terrain_set, int p_terrain_index) const {
	ERR_FAIL_INDEX_V(p_terrain_set, terrain_sets.size(), String());
	ERR_FAIL_INDEX_V(p_terrain_index, terrain_sets[p_terrain_set].terrains.size(), String());
	return terrain_sets[p_terrain_set].terrains[p_terrain_index].name;
}
void TileSet::set_terrain_color(int p_terrain_set, int p_terrain_index, Color p_color) {
	ERR_FAIL_INDEX(p_terrain_set, terrain_sets.size());
	ERR_FAIL_INDEX(p_terrain_index, terrain_sets[p_terrain_set].terrains.size());
	if (p_color.a != 1.0) {
		WARN_PRINT("Terrain color should have alpha == 1.0");
		p_color.a = 1.0;
	}
	terrain_sets.write[p_terrain_set].terrains.write[p_terrain_index].color = p_color;
	emit_changed();
}
Color TileSet::get_terrain_color(int p_terrain_set, int p_terrain_index) const {
	ERR_FAIL_INDEX_V(p_terrain_set, terrain_sets.size(), Color());
	ERR_FAIL_INDEX_V(p_terrain_index, terrain_sets[p_terrain_set].terrains.size(), Color());
	return terrain_sets[p_terrain_set].terrains[p_terrain_index].color;
}
bool TileSet::is_valid_terrain_peering_bit_for_mode(TileSet::TerrainMode p_terrain_mode, TileSet::CellNeighbor p_peering_bit) const {