build: Add build target for vkcube-display

[enunes]

Add a new CMake target for vkcube-display, which the variant using VK_KHR_display.
This is a variant which already exists, but is currently hard to build as only one variant can be selected per build using CUBE_WSI_SELECTION. With this option, it is possible to build this variant in addition to other ones, similar to what is done for vkcube-wayland.

As of now this only takes effect in Linux (same as the wayland one below it) as this is the only platform I'm able to test on.

[ci-tester-lunarg]

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[ci-tester-lunarg]

Author enunes not on autobuild list. Waiting for curator authorization before starting CI build.

[CLAassistant]

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[charles-lunarg]

I want to state that while this PR is welcome, the solution is more of a band-aid than a long term fix. The root cause is vkcube being unable to choose the WSI platform at runtime.
I am already working on a PR which enables selection of xcb, xlib, wayland, directfb, and this PR's VK_KHR_display. I'd appreciate your review when I publish it in the coming days. I have the loading all happening, the last bit of work is polishing the selection logic to handle automatically choose an available WSI platform.

[enunes]

I agree that a refactor is better either by reworking the CMake logic or choosing the WSI at runtime. If you're already working on it then that will be better.

So I can help review the new PR when it arrives and maybe we can drop this one.
In the end all I'm looking for is a way to ship this cleanly in distributions so I don't have to build it over and over for a quick check for development on new installs. If the new PR would still take a long time, then I would still be interested in having this as a way to build this for distributions in the meanwhile.

[charles-lunarg]

Closing in favor of #1029

[charles-lunarg]

In the end all I'm looking for is a way to ship this cleanly in distributions so I don't have to build it over and over for a quick check for development on new installs. If the new PR would still take a long time, then I would still be interested in having this as a way to build this for distributions in the meanwhile.

That is exactly the reason that vkcube should have supported multiple WSI platforms at the same time, but alas it was easier to just copy-paste the cmake code and tweak the build than it was to actually make it work.
It has irked me that vkcube didn't handle that, so thank you for making this PR in order to encourage me to fix the underlying problem :D

[enunes]

@charles-lunarg Thanks! I'll test the new PR and post some review feedback there shortly.