### 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)

if (tile_meshes_dirty) {
		Vector<Vector2> shape = get_tile_shape_polygon();
		Vector<Vector2> uvs;
		uvs.resize(shape.size());
		for (int i = 0; i < shape.size(); i++) {
			uvs.write[i] = shape[i] + Vector2(0.5, 0.5);
		}
		Vector<Color> colors;
		colors.resize(shape.size());
		colors.fill(Color(1.0, 1.0, 1.0, 1.0));
		
		tile_filled_mesh->clear_surfaces();
		Array a;
		a.resize(Mesh::ARRAY_MAX);
		a[Mesh::ARRAY_VERTEX] = shape;
		a[Mesh::ARRAY_TEX_UV] = uvs;
		a[Mesh::ARRAY_COLOR] = colors;
		a[Mesh::ARRAY_INDEX] = Geometry2D::triangulate_polygon(shape);
		tile_filled_mesh->add_surface_from_arrays(Mesh::PRIMITIVE_TRIANGLES, a, Array(), Dictionary(), Mesh::ARRAY_FLAG_USE_2D_VERTICES);
		
		tile_lines_mesh->clear_surfaces();
		a.clear();
		a.resize(Mesh::ARRAY_MAX);
		
		shape.push_back(shape[0]);
		colors.push_back(colors[0]);
		a[Mesh::ARRAY_VERTEX] = shape;
		a[Mesh::ARRAY_COLOR] = colors;