### 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_MSG(!tiles.has(p_atlas_coords), Rect2i(), vformat("TileSetAtlasSource has no tile at %s.", String(p_atlas_coords)));
	ERR_FAIL_INDEX_V(p_frame, (int)tiles[p_atlas_coords].animation_frames_durations.size(), Rect2i());
	const TileAlternativesData &tad = tiles[p_atlas_coords];
	Vector2i size_in_atlas = tad.size_in_atlas;
	Vector2 region_size = texture_region_size * size_in_atlas + separation * (size_in_atlas - Vector2i(1, 1));
	Vector2i frame_coords = p_atlas_coords + (size_in_atlas + tad.animation_separation) * ((tad.animation_columns > 0) ? Vector2i(p_frame % tad.animation_columns, p_frame / tad.animation_columns) : Vector2i(p_frame, 0));
	Vector2 origin = margins + (frame_coords * (texture_region_size + separation));
	return Rect2(origin, region_size);
}
bool TileSetAtlasSource::is_position_in_tile_texture_region(const Vector2i p_atlas_coords, int p_alternative_tile, Vector2 p_position) const {