### 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_MSG(p_coords.x < 0 || p_coords.y < 0, vformat("Cannot set cell with negative coords in a TileMapPattern. Wrong coords: %s", p_coords));
	size = size.max(p_coords + Vector2i(1, 1));
	pattern[p_coords] = TileMapCell(p_source_id, p_atlas_coords, p_alternative_tile);
	emit_changed();
}
bool TileMapPattern::has_cell(const Vector2i &p_coords) const {
	return pattern.has(p_coords);
}
void TileMapPattern::remove_cell(const Vector2i &p_coords, bool p_update_size) {
	ERR_FAIL_COND(!pattern.has(p_coords));
	pattern.erase(p_coords);
	if (p_update_size) {
		size = Size2i();
		for (const KeyValue<Vector2i, TileMapCell> &E : pattern) {
			size = size.max(E.key + Vector2i(1, 1));
		}
	}
	emit_changed();
}
int TileMapPattern::get_cell_source_id(const Vector2i &p_coords) const {