mac/vulkan: error out on context creation without an NSApplication

if no NSApplication has been initialized, applications using Appkit
functionality are not supposed to work properly or just deadlock
indefinitely. properly error out on macvk context creation in that case.

Author: Akemi

video/out/vulkan/context_mac.m

@@ -56,6 +56,11 @@ static bool mac_vk_init(struct ra_ctx *ctx)
     struct mpvk_ctx *vk = &p->vk;
     int msgl = ctx->opts.probing ? MSGL_V : MSGL_ERR;
 
+    if (!NSApp) {
+        MP_ERR(ctx, "Failed to initialise macvk context, no NSApplication initialized.\n");
+        goto error;
+    }
+
     if (!mpvk_init(vk, ctx, VK_EXT_METAL_SURFACE_EXTENSION_NAME))
         goto error;