demos: Use correct type for alphaMode in display surface
demos: Use correct type for alphaMode in display surface

alphaMode in VkDisplaySurfaceCreateInfoKHR is
VkDisplayPlaneAlphaFlagBitsKHR, not VkCompositeAlphaFlagBitsKHR.

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools