[host] allow user to select capture backend

This commit introduces a new option, app:capture, which can be set to
either DXGI or NvFBC to force the host application to use that backend.

This is very useful for testing DXGI on Quadro cards, which would default
to running with NvFBC.

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

@@ -235,6 +235,7 @@ static CaptureResult xcb_getFrame(FrameBuffer * frame)
 
 struct CaptureInterface Capture_XCB =
 {
+  .shortName       = "XCB",
   .getName         = xcb_getName,
   .create          = xcb_create,
   .init            = xcb_init,