Save previously generated blocks on Mapgen blitback
Update Mapgen VoxelManipulator on buffer invalidation
Add LuaVoxelManip methods: get_node_at() and set_node_at()
Add liquid thread
Don't save alpha channel in screenshots (fixes #1451)
Merge remote-tracking branch 'minetest/master'

Conflicts:
	src/environment.cpp
	src/map.cpp
Change screenshot colorformat properly
Merge remote-tracking branch 'minetest/master'
Merge remote-tracking branch 'upstream/master' into next