This commit adds a new rendering backend, GLES2, and adds a
project setting to enable it.
Currently this backend can only be used on the X11 platform,
but integrating into other platforms is planned.
The reordering of the SConscript includes allows to ensure that
stuff like the builtin zlib headers will be available for libpng.
Also moved glew back into global env, otherwise windows seems
not to find it... Kind of shooting in the dark with this multi-env
setup.