vulkan: Actually return the preferred memory format

We were just exiting the loop, but not remembering the index.

Speeds up various memory operations, sometimes by quite a lot.

gsk/gpu/gskvulkandevice.c

@@ -1076,8 +1076,11 @@ gsk_vulkan_device_find_allocator (GskVulkanDevice       *self,
       found = MIN (i, found);
 
       if ((properties.memoryTypes[i].propertyFlags & desired_flags) == desired_flags)
+        {
+          found = i;
           break;
         }
+  }
 
   g_assert (found < properties.memoryTypeCount);