libs: backends: Set errno to EINVAL if backend params are wrong.

This makes the python trace understandable.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
backends: SDL: Fix resize.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
demos: py_simple: Add BackendInit() example.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
doc: Start python backends docs.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
pywrap: backends: Hack typemap for the BackendInit(..., FILE*)

The Python 3 no longer uses FILE * for file object so we
hack around that by creating temporal FILE* from the underlying
file descriptor.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
demos: py_simple: Pass the stderr as the last parameter.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
doc: Update python backend docs.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>