### 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 (p_to_pos >= 0 && p_to_pos <= terrain_set) {
		terrain_set += 1;
	}
}
void TileData::move_terrain_set(int p_from_index, int p_to_pos) {
	if (p_from_index == terrain_set) {
		terrain_set = (p_from_index < p_to_pos) ? p_to_pos - 1 : p_to_pos;
	} else {
		if (p_from_index < terrain_set) {
			terrain_set -= 1;
		}
		if (p_to_pos <= terrain_set) {
			terrain_set += 1;
		}
	}
}
void TileData::remove_terrain_set(int p_index) {
	if (p_index == terrain_set) {
		terrain_set = -1;
		for (int i = 0; i < 16; i++) {
			terrain_peering_bits[i] = -1;
		}
	} else if (terrain_set > p_index) {
		terrain_set -= 1;
	}
}
void TileData::add_terrain(int p_terrain_set, int p_to_pos) {