add shader quad control sample demonstrating VK_KHR_shader_quad_control features.

[gpx1000]

Description

This adds in the shader quad control sample.

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• My code follows the coding style
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[asuessenbach]

Just one minor issue on source level.

Could you add a picture how it's supposed to look like?
I'm getting a window with seemingly perfectly smooth color gradient.

[gary-sweet]

All I can say is that this correctly reports as unsupported for me.