Add new, longer 'default_grass_side' texture

Update side texture to match the new top texture.
3/4 side coverage instead of 1/2,

mods/default/README.txt

@@ -101,7 +101,7 @@ paramat (CC BY-SA 3.0):
   default_dry_grass.png
   default_dry_grass_side.png
   default_dry_grass_*.png
-  default_grass_side.png
+  default_grass_side.png -- Derived from a texture by TumeniNodes (CC-BY-SA 3.0)
   default_mese_block.png
   default_silver_sand.png
   default_mese_post_light_side.png