### 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 = _get_isometric_terrain_polygon(tile_size);
			} else {
				float overlap = 0.0;
				switch (tile_shape) {
					case TileSet::TILE_SHAPE_HEXAGON:
						overlap = 0.25;
						break;
					case TileSet::TILE_SHAPE_HALF_OFFSET_SQUARE:
						overlap = 0.0;
						break;
					default:
						break;
				}
				polygon = _get_half_offset_terrain_polygon(tile_size, overlap, tile_offset_axis);
			}
			{
				Ref<ArrayMesh> mesh;
				mesh.instantiate();
				Vector<Vector2> uvs;
				uvs.resize(polygon.size());
				Vector<Color> colors;
				colors.resize(polygon.size());
				colors.fill(Color(1.0, 1.0, 1.0, 1.0));
				Array a;
				a.resize(Mesh::ARRAY_MAX);
				a[Mesh::ARRAY_VERTEX] = polygon;
				a[Mesh::ARRAY_TEX_UV] = uvs;
				a[Mesh::ARRAY_COLOR] = colors;
				a[Mesh::ARRAY_INDEX] = Geometry2D::triangulate_polygon(polygon);
				mesh->add_surface_from_arrays(Mesh::PRIMITIVE_TRIANGLES, a, Array(), Dictionary(), Mesh::ARRAY_FLAG_USE_2D_VERTICES);