### 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), 1, vformat("TileSetAtlasSource has no tile at %s.", Vector2i(p_atlas_coords)));
	return tiles[p_atlas_coords].animation_frames_durations.size();
}
void TileSetAtlasSource::set_tile_animation_frame_duration(const Vector2i p_atlas_coords, int p_frame_index, real_t p_duration) {
	ERR_FAIL_COND_MSG(!tiles.has(p_atlas_coords), vformat("TileSetAtlasSource has no tile at %s.", Vector2i(p_atlas_coords)));
	ERR_FAIL_INDEX(p_frame_index, (int)tiles[p_atlas_coords].animation_frames_durations.size());
	ERR_FAIL_COND(p_duration <= 0.0);
	tiles[p_atlas_coords].animation_frames_durations[p_frame_index] = p_duration;
	emit_signal(SNAME("changed"));
}
real_t TileSetAtlasSource::get_tile_animation_frame_duration(const Vector2i p_atlas_coords, int p_frame_index) const {