Add transition terrain surfaces for fir needles and short grass

[Kromster80]

I love there are Dirt and yellow, DirtStones10, DirtStones50 and other similar "mixed" textures. They help with blending the grass, dirt and stone patches together really, really well.

Sadly, we don't have textures like that with Fir needles and with Grass short. Currently, the Grass short and Fir needles textures are hard to use, because they feel really out of place next to almost anything else. Some blended mixed textures, like Fir 50 on dirt, Fir yellow 50 on dirt, or Grass short 50 on dirt, would be really helpful.
// Amaroth