Font attributes do not persist when fallback font is used

### Problem description
If I define a font with the following string:
```
bold 16px InvalidFontName, Helvetica, Arial, sans-serif;
```
as a result, **non-bold** Helvetica will be used.

### Expected behavior

Font attributes should be preserved when the fallback font is used.

### Environment

- Tabris.js version: 3.2.1
- OS: iOS (12 & 13)

### Code snippet

```javascript
import {TextView, contentView, Stack, Font} from 'tabris';

const boldHelvetica = Font.from("bold 16px Helvetica, Arial, sans-serif;");
const invalidFont = Font.from("bold 16px InvalidFontName, Helvetica, Arial, sans-serif;");
const helvetica = Font.from("16px Helvetica, Arial, sans-serif;");

contentView.append(





);
```