introduce the EON_Rend structure.
remove private structures.
add Clip data structure.
port to the new EON_Rend API.

move all the globals in a context, made the code (more) reentrant
and thread-safe. Still not finished, though.
port to the new EON_Clip API.

still work in progress, however we're almost reentrant
and thread-aware.
prepare the face sorting code for globals removal.
require Z-Buffer.

so we can remove the polygon-sorting code which is
ridden with globals and hidden static state.
Merge branch 'rendcontext'

remove hidden/global/static state and make the core renderer
reentrant and thread-aware.