nitg: fix C return type of callbacks to Nit

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg: fix FFI error with arguments name of extern methods

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg-s: fix importation of custom C header between modules

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg-s: all extern type use the C structures of kernel::Pointer

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg-e: all extern type use the C structures of kernel::Pointer

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg: activate FFI in all engines

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
tests: skip test_json in nitg-s

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
tests: do not skip opengles2_hello_triangle in nitg-s

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg-s: fix "No provided declaration" with FFI casts

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>