Improvements to Forest Tileset (especially underground)

[Karkus476]

We need some better tiles, for the background of the forest world, especially when underground. Some levels still use image backgrounds, but we have parallax scrolling, so we should use it

[Hume2]

Could you provide a list of what all tiles are requested?

[Grumbel]

There is a list in the Wiki:

https://github.com/SuperTux/supertux/wiki/TODO#graphics