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()
.
Follow-up to !3333 (merged)