build: Disable asserts and cast checks

For non-debug and non-plain cases - i.e. mainly release builds - in
order to mirror Mutter.

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/gnome-shell/-/merge_requests/3099>

meson.build

@@ -225,6 +225,11 @@ if buildtype != 'plain'
     gs_c_args += [
       '-DG_ENABLE_DEBUG',
     ]
+  else
+    gs_c_args += [
+      '-DG_DISABLE_ASSERT',
+      '-DG_DISABLE_CAST_CHECKS',
+    ]
   endif
 
   supported_gs_c_args = cc.get_supported_arguments(gs_c_args)