Add TextureUnmanaged

[RobLoach]

@Ataraxia-Mechanica What about something like this?

raylib::Font font("myfont.ttf");
{
  raylib::TextureUnmanaged fontTexture = font.GetTexture();
  fontTexture.Draw();
}
// No more error since we're usning a Unmanaged object?

[Ataraxia-Mechanica]

This looks pretty cool, it allows method chaining too which is also a plus

[RobLoach]

What do you think ab ok it having TextureUjmanaged inherit from Texture instead ifnyhe other way around?

[Ataraxia-Mechanica]

I think both way's fine

[RobLoach]

Thinking about this more, I think having this inheitance structure makes the most sense...

Texture
- raylib::TextureUnmanaged
  - raylib::Texture

Then the majority of work is in the TextureUnmanaged classes, and the Texture just adds the managed aspect, along with the deleted copy constructor, etc.