vkcube should treat VK_KHR_display as runtime command-line switch, not as cmake option

[jjulianoatnv]

The way that vkcube is structured today, the choice to use VK_KHR_display to drive a direct-to-display is a cmake option when configuring the build.

if(UNIX AND NOT APPLE) # i.e. Linux
...
    set(CUBE_WSI_SELECTION "XCB" CACHE STRING "Select WSI target for vkcube (XCB, XLIB, WAYLAND, DISPLAY)")

This means a separate vkcube binary is required in order to test VK_KHR_display.

It would be better if the choice between windowing system swapchain and VK_KHR_display swapchain was controlled by a runtime switch specified on the command line. This issue requests that the choice of DISPLAY be moved to a command line switch.

Extrapolating, the choice between XCB and XLIB and WAYLAND could also become command-line options; however I am not requesting such a change in this issue. If there is desire to go this extra distance then it can be done in the context of a different gitlab issue.

Further, by placing the DISPLAY support inside of a check for UNIX, this precludes ever exposing VK_KHR_display on Windows or on any other non-UNIX platform. That is short-sighted. A better software architecture would structure the code such that support for DISPLAY is independent of the windowing system choice.