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