### 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)

Vector2 unit = Vector2(p_size) / 6.0;
	Vector<Vector2> polygon;
	switch (p_bit) {
		case TileSet::CELL_NEIGHBOR_RIGHT_CORNER:
			polygon.push_back(Vector2(0.5, -0.5) * unit);
			polygon.push_back(Vector2(1.5, -1.5) * unit);
			polygon.push_back(Vector2(3, 0) * unit);
			polygon.push_back(Vector2(1.5, 1.5) * unit);
			polygon.push_back(Vector2(0.5, 0.5) * unit);
			polygon.push_back(Vector2(1, 0) * unit);
			break;