### 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)

for (int i = 0; i < TileSet::CELL_NEIGHBOR_MAX; i++) {
		if (is_valid_bit[i] != p_terrains_pattern.is_valid_bit[i]) {
			return is_valid_bit[i] < p_terrains_pattern.is_valid_bit[i];
		}
	}
	if (terrain != p_terrains_pattern.terrain) {
		return terrain < p_terrains_pattern.terrain;
	}
	for (int i = 0; i < TileSet::CELL_NEIGHBOR_MAX; i++) {
		if (is_valid_bit[i] && bits[i] != p_terrains_pattern.bits[i]) {
			return bits[i] < p_terrains_pattern.bits[i];
		}
	}
	return false;
}
bool TileSet::TerrainsPattern::operator==(const TerrainsPattern &p_terrains_pattern) const {
	for (int i = 0; i < TileSet::CELL_NEIGHBOR_MAX; i++) {
		if (is_valid_bit[i] != p_terrains_pattern.is_valid_bit[i]) {
			return false;
		}
		if (is_valid_bit[i] && bits[i] != p_terrains_pattern.bits[i]) {
			return false;
		}
	}
	if (terrain != p_terrains_pattern.terrain) {
		return false;
	}
	return true;
}
void TileSet::TerrainsPattern::set_terrain(int p_terrain) {