Xnest: tidy up extension blacklisting in miinitext.c
The DDX'es sometimes need to disable certain extensions. Instead of complex include cascades with ifdef'ed ddx-specific include from dix code, it's more clean to add some clear and explicit knobs set by the DDX'es individual meson.build.
Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net