GPU: Add Dummy Backend For Unsupported Platforms

With the introduction of metal and vulkan we use a different
GPU backend selection which broke the dialog box for unsupported
platforms.

Blender asserted and segfaulted before the dialog was being shown
to the user.

This patch solves this by introducing a dummy GPU backend in case no
GPU backend was supported by OpenGL, Metal and Vulkan Backend.

@@ -2016,0 +2022,4 @@
+                                                 GHOST_DialogOptions dialog_options) const
+{
+  @autoreleasepool {
+    NSAlert *alert = [[[NSAlert alloc] init] autorelease];

@@ -2590,3 +2590,2 @@
   config.pszContent = message_16;
-  config.pButtons = (link) ? buttons : buttons + 1;
-  config.cButtons = (link) ? 2 : 1;
+  config.pButtons = strlen(link) ? buttons : buttons + 1;