### 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 (has_source(a[0]) && get_source(a[0])->has_tile(a[1]) && get_source(a[0])->has_alternative_tile(a[1], a[2])) {
			alternative_to_remove.push_back(a);
		}
	}
	for (int i = 0; i < alternative_to_remove.size(); i++) {
		Array a = alternative_to_remove[i];
		remove_alternative_level_tile_proxy(a[0], a[1], a[2]);
	}
}
void TileSet::clear_tile_proxies() {
	source_level_proxies.clear();
	coords_level_proxies.clear();
	alternative_level_proxies.clear();
	emit_changed();
}
int TileSet::add_pattern(Ref<TileMapPattern> p_pattern, int p_index) {
	ERR_FAIL_COND_V(!p_pattern.is_valid(), -1);
	ERR_FAIL_COND_V_MSG(p_pattern->is_empty(), -1, "Cannot add an empty pattern to the TileSet.");
	for (const Ref<TileMapPattern> &pattern : patterns) {
		ERR_FAIL_COND_V_MSG(pattern == p_pattern, -1, "TileSet has already this pattern.");
	}
	ERR_FAIL_COND_V(p_index > (int)patterns.size(), -1);
	if (p_index < 0) {
		p_index = patterns.size();
	}
	patterns.insert(p_index, p_pattern);
	emit_changed();
	return p_index;
}
Ref<TileMapPattern> TileSet::get_pattern(int p_index) {