Added SDL_assert_always (never disabled).

include/SDL_assert.h

@@ -165,6 +165,9 @@ extern DECLSPEC SDL_assert_state SDLCALL SDL_ReportAssertion(SDL_assert_data *,
 #   error Unknown assertion level.
 #endif
 
+/* this assertion is never disabled at any level. */
+#define SDL_assert_always(condition) SDL_enabled_assert(condition)
+
 
 typedef SDL_assert_state (SDLCALL *SDL_AssertionHandler)(
                                  const SDL_assert_data* data, void* userdata);