Make cancel_callback check duplicates luaL_unref only in debug mode.
Divide static tiles so that they always have the size of their pattern.

This allows to optimize away more tiles when creating the list of tiles
in animated regions.
Indeed, tiles expanded in big rectangles like a lake or a dungeon floor
were entirely redrawn at each frame if just one small animated tile
overlapped them.
Remove costly conversions from const char* to std::string.
Kill dead code.
Speed up LuaContext::push_userdata().

Time was wasted building assertion error strings.
Merge branch 'master' into texture

Conflicts:
	include/lua/LuaContext.h
	src/lua/LuaContext.cpp