cursor-tracker: Use a separate cursor for xfixes cursor tracking
cursor-tracker: Remove the grab cursor

We expect that meta_screen_set_cursor while grabbed will properly
set the cursor on the root window. Make sure this works by simply
always using the root cursor when we have an active grab.
backend-x11: Always pass None for the cursor

This means we fall back to what's defined on the root window, which
will give us our properly set cursor.