meson: set G_DISABLE_CAST_CHECKS unless debugging is enabled

G_DISABLE_CAST_CHECKS gets set with buildtype=release but not with buildtype=plain (usually used by distros) which is confusing for downstream since it's not clear what GTK upstream recommends for a "release build" and if it should be set by distros when plain is used.

Instead just set it when debug=false, independent of the optimization level, so plain and release get the same defines.

The old autotools build also set it by default when not passing a debug level, so nothing should depend on it. The default buildtype is debugoptimized, so nothing changes there.