Bugfix: Set the cursor via the Xlib connection if XCursor is supported

Bugfix: Set the cursor via the Xlib connection if XCursor is supported

This fixes a race where we created cursors on the Xlib connection, flushed,
then used the cursor on the XCB connection. Even though we flushed, the X
server did not process the requests yet and therefore returned a BadCursor
error.

This bugfix uses the Xlib connection for setting the root window cursor which
will ensure that the requests are properly serialized.

An easy test for this (on my machine) is the following ~/.xsession:
    xsetroot -cursor_name cross
    exec i3
If you see a cross cursor instead of the pointer, the race happens. You’ll see
a error_code=6 error in your ~/.xsession-errors.