You can now build Filament with support for both X11 APIs, or neither.
If both are supported, run-time selection is achieved using a SwapChain
flag.
Supporting only one API at build time (or neither) is useful because our
list of "required" VkInstance extensions can vary according to which
API's are supported. During VkInstance creation, we do not have a priori
knowledge about what kinds of swap chains will be created. (headless vs
non-headless, XCB vs XLIB, etc)
Note that some Vulkan implementation (e.g. some builds of SwiftShader)
only support XCB.
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