Fix parsing of italic correction

`tftopl.web` includes:

```
@d height_index(#)==(tfm[char_info(#)+1] div 16)
@d depth_index(#)==(tfm[char_info(#)+1] mod 16)
@d italic_index(#)==(tfm[char_info(#)+2] div 4)
```

This change updates the italic correction parsing to divide by 4 properly (right-shift 2) instead of the incorrect division by 64 (right-shift 6) that it did previously. (The masks match the bit lengths of each field; tftopl specifies 8 bits for width, 4 for height, 4 for depth, 6 for italic, 2 for tag, and 8 for remainder.)