Misc: Added an explicit compile-time test for non-scoped IM_ASSERT() macros to redirect users to a solution + fixed our stb wrappers.

+ Nav: Use nav layer enum, comments.

misc/freetype/imgui_freetype.cpp

@@ -280,7 +280,7 @@ namespace
 
 #ifndef STB_RECT_PACK_IMPLEMENTATION                        // in case the user already have an implementation in the _same_ compilation unit (e.g. unity builds)
 #ifndef IMGUI_DISABLE_STB_RECT_PACK_IMPLEMENTATION
-#define STBRP_ASSERT(x)     IM_ASSERT(x)
+#define STBRP_ASSERT(x)     do { IM_ASSERT(x); } while (0)
 #define STBRP_STATIC
 #define STB_RECT_PACK_IMPLEMENTATION
 #endif