build: Disable asserts and cast checks in Mutter

For non-debug and non-plain cases - i.e. mainly release builds.

This ensures we use the same options in all places and draws a cleaner
distinction between g_assert() and g_return_if_fail() - the later will
still be done in release build while the former are meant for debug
only.

One advantage of doing this is that it allows us to use non-trivial
asserts more generously, such as calling `g_list_length()`.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3424>

meson.build

@@ -519,6 +519,11 @@ if buildtype != 'plain'
     mutter_c_args += [
       '-DG_ENABLE_DEBUG',
     ]
+  else
+    mutter_c_args += [
+      '-DG_DISABLE_ASSERT',
+      '-DG_DISABLE_CAST_CHECKS',
+    ]
   endif
 
   supported_mutter_c_args = cc.get_supported_arguments(mutter_c_args)