### 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_COND_V(p_terrain_set < 0 || p_terrain_set >= get_terrain_sets_count(), Vector<Point2>());
	TileSet::TerrainMode terrain_mode = get_terrain_set_mode(p_terrain_set);
	if (tile_shape == TileSet::TILE_SHAPE_SQUARE) {
		if (terrain_mode == TileSet::TERRAIN_MODE_MATCH_CORNERS_AND_SIDES) {
			return _get_square_corner_or_side_terrain_peering_bit_polygon(tile_size, p_bit);
		} else if (terrain_mode == TileSet::TERRAIN_MODE_MATCH_CORNERS) {
			return _get_square_corner_terrain_peering_bit_polygon(tile_size, p_bit);
		} else { 
			return _get_square_side_terrain_peering_bit_polygon(tile_size, p_bit);
		}
	} else if (tile_shape == TileSet::TILE_SHAPE_ISOMETRIC) {
		if (terrain_mode == TileSet::TERRAIN_MODE_MATCH_CORNERS_AND_SIDES) {
			return _get_isometric_corner_or_side_terrain_peering_bit_polygon(tile_size, p_bit);
		} else if (terrain_mode == TileSet::TERRAIN_MODE_MATCH_CORNERS) {
			return _get_isometric_corner_terrain_peering_bit_polygon(tile_size, p_bit);
		} else { 
			return _get_isometric_side_terrain_peering_bit_polygon(tile_size, p_bit);
		}
	} else {
		float overlap = 0.0;
		switch (tile_shape) {
			case TileSet::TILE_SHAPE_HEXAGON: