Add grass again

[EmiOnGit]

Adds a patch of grass using the warbler_grass crate :)

The 'controversial' decisions I made:

• The 'Ground' mesh is used as a base for the grass field (only checked once).
• A low-res density texture is used for marking the points, where grass should be found. (assets/textures/grass_density_map.png).
• The density texture is not hot swappable at runtime to preserve a bit of performance.
• The grass is configured to be brownish at the bottom to resemble the ground a bit more.
• The grass is a bit higher than usual since I think it looks a bit odd if the grass has the exact same height as the player.

[janhohenheim]

Thanks a bunch :D I'm so happy about seeing those blades of grass again, haha
Only issue I have is that this makes the ground "glossy" by removing the hardcoded reflectance. Let me fix that real quick.

[EmiOnGit]

Oh I didn't notice that I made the ground more glossy. Good catch! :)