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.
This allows clients to build the Vulkan backend with either XLIB or XCB
support. I did a quick smoke test of the XCB option, but for now we are
continuing to default to XLIB. Note that the native window type used to
create the swap chain differs between these two API's.
