### 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)

polygon.push_back(Vector2(0, -1) * unit);
			break;
		default:
			break;
	}
	return polygon;
}
Vector<Point2> TileSet::_get_half_offset_terrain_polygon(Vector2i p_size, float p_overlap, TileSet::TileOffsetAxis p_offset_axis) {
	Vector2 unit = Vector2(p_size) / 6.0;
	if (p_offset_axis == TileSet::TILE_OFFSET_AXIS_HORIZONTAL) {
		return {
			Vector2(1, 1.0 - p_overlap * 2.0) * unit,
			Vector2(0, 1) * unit,
			Vector2(-1, 1.0 - p_overlap * 2.0) * unit,
			Vector2(-1, -1.0 + p_overlap * 2.0) * unit,
			Vector2(0, -1) * unit,
			Vector2(1, -1.0 + p_overlap * 2.0) * unit,
		};
	} else {
		return {
			Vector2(1, 0) * unit,
			Vector2(1.0 - p_overlap * 2.0, -1) * unit,
			Vector2(-1.0 + p_overlap * 2.0, -1) * unit,
			Vector2(-1, 0) * unit,
			Vector2(-1.0 + p_overlap * 2.0, 1) * unit,
			Vector2(1.0 - p_overlap * 2.0, 1) * unit,
		};
	}
}
Vector<Point2> TileSet::_get_half_offset_corner_or_side_terrain_peering_bit_polygon(Vector2i p_size, float p_overlap, TileSet::TileOffsetAxis p_offset_axis, TileSet::CellNeighbor p_bit) {