fix to blitbuffer.lua, adaption of Lua/C-API

changes the Lua/C-API to accept both "blitbuffer" userdata as created by the "classic" Lua/C API (blitbuffer.c) and lightuserdata which it gets passed when called with FFI-created "BlitBuffer" structs as parameters.

also adds rounding to the blitting functions since these may get called with non-integer values from other Lua code