Minimal game: add /dummyball <count> command
Add video driver selection to settings menu (based uppon idea from webdesigner97)
Fix seg fault if popping from empty stack (L-system trees)

See: https://github.com/minetest/minetest/issues/1525

Background
Wuzzy2: If you attempt to spawn a L-system tree with minetest.spawn_tree, you can make Minetest crash if it is attempted to pop an empty stack.

ShadowNinja: This shouldn't cause a segmentation fault, but it should throw a Lua error

Commit Description
This commit throws a Lua error instead of causing a segmentation fault. The server will still "crash" but will include a Lua backtrace.

L-Systems fix randomness
Unless a random seed is provided (via Lua treedef) seed the PRNG with a different seed for each tree
Resolves: https://github.com/minetest/minetest/issues/1469

Fix l-system crash when treedef random_level not set by Lua
Fix LuaJIT exception wrapper
Add lua exception handling test code
Catch some error situations when mod used without thinking about it
Fix retval of entity.get_staticdata beeing lost while profiling is enabled
Allow use all 6 faces for special tiles.
Protocol version bump.
Add missing doc for minetest.get_us_time()
Remove invalid doc about minetest.get_time_us()
Fix interlaced 3d mode second image being flipped on compiling against irrlicht >= 1.8
Fix only one texture being updated on window resize breaking sidebyside and topbottom 3d modes
Save previously generated blocks on Mapgen blitback
Update Mapgen VoxelManipulator on buffer invalidation
Add LuaVoxelManip methods: get_node_at() and set_node_at()
Don't save alpha channel in screenshots (fixes #1451)
Change screenshot colorformat properly
Fix access to invalid data on reception of packet with size 0
Log unhandled exceptions in connectionthreads to errorstream
Fix typo in default configuration file
Fix another typo in example configuration