Minimal: Add snow biome and jungleleaves nodes. Add mapgen aliases

games/minimal/mods/default/init.lua

games/minimal/mods/default/mapgen.lua

games/minimal/mods/default/textures/default_ice.png

games/minimal/mods/default/textures/default_jungleleaves.png

games/minimal/mods/default/textures/default_pine_needles.png

games/minimal/mods/default/textures/default_pinetree.png

games/minimal/mods/default/textures/default_pinetree_top.png

games/minimal/mods/default/textures/default_snow.png

games/minimal/mods/default/textures/default_snow_side.png

games/minimal/mods/default/textures/default_snowball.png

src/treegen.cpp