demos:Add cube option --validate-checks-disabled
demos:Add cube option --validate-checks-disabled

Added option to cube demo "--validate-checks-disabled".
This option will use the VK_EXT_validation_flags extension to cause
validation to skip all validation flags that have a flag.
This is meant to provide a basic test for the VK_EXT_validation_flags
extension and find any obvious bugs/crashes.

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