Underlined text not being customizable in resource packs

The underline in underlined text cannot be changed in resource packs, resulting in custom fonts looking bad with underline. It should use a texture so that it can be changed.

To Reproduce:
1. Download the example resource pack
2. Type in this command: /tellraw @a {"text":"Example Underlined Text","underlined":true}
3. The underline is too thick and it cannot be changed