[host] app: allow the capture interface to select async or sync mode

While it's correct for DXGI to use a asyncronous waitFrame model, other
capture interfaces such as NvFBC it is not correct. This change allows
the capture interface to specify which is more correct for it and moves
the waitFrame/post into the main thread if async is not desired.

host/platform/Linux/capture/XCB/src/xcb.c

@@ -236,6 +236,7 @@ static CaptureResult xcb_getFrame(FrameBuffer * frame, const unsigned int height
 struct CaptureInterface Capture_XCB =
 {
   .shortName       = "XCB",
+  .asyncCapture    = true,
   .getName         = xcb_getName,
   .create          = xcb_create,
   .init            = xcb_init,