EMF_USE_MIP_MAPS was introduced in Irrlicht 1.8, don't use in 1.7.3 or below

This is not optimal as it will cause minor graphical glitches for
Irrlicht 1.7 users, but at least it fixes the build.
Wield: always obey anisotropic_filter setting, not just for hi res textures
Don't corrupt stepheight when setting other properties
Add minetest.copy_table(table) To get rid off the "table references"

Signed-off-by: Craig Robbins <kde.psych@gmail.com>
Add extra lua doc for remove_item

This isn't as silly as it sounds: iterate a list containing items with
unique metadata each, and remove_item the first one you find, placing
into a different - fine, except the remove will invariable remove the
wrong one, leading to items being duplicated and others destroyed.

Arguably it's a bug, and Inventory::removeItem should actually remove
the item you tell it to. (i.e. if the passed ItemStack has metadata,
match it). But it seems reasonable to just document the behaviour
more clearly.
Improve string_to_pos documentation
Minor lua api doc fix
Clarify dig_node documentation (return value)
Better on_step documentation