wayland: Check that all required dynamic symbols have been successfully resolved at init time

Ensure that all hard dependencies are resolved when dynamically loading the libraries required for the Wayland backend and fail gracefully if a required module was not initialized successfully.

src/video/wayland/SDL_waylanddyn.c

@@ -152,8 +152,11 @@ int SDL_WAYLAND_LoadSymbols(void)
 #define SDL_WAYLAND_INTERFACE(iface)        WAYLAND_##iface = (struct wl_interface *)WAYLAND_GetSym(#iface, thismod, SDL_TRUE);
 #include "SDL_waylandsym.h"
 
-        if (SDL_WAYLAND_HAVE_WAYLAND_CLIENT) {
-            /* all required symbols loaded. */
+        if (SDL_WAYLAND_HAVE_WAYLAND_CLIENT &&
+            SDL_WAYLAND_HAVE_WAYLAND_CURSOR &&
+            SDL_WAYLAND_HAVE_WAYLAND_EGL &&
+            SDL_WAYLAND_HAVE_WAYLAND_XKB) {
+            /* All required symbols loaded, only libdecor is optional. */
             SDL_ClearError();
         } else {
             /* in case something got loaded... */