The window ID includes the client ID anyway, so we might as well just look up
the client directly instead of trying to get the window first and the client
from the window.
This also fixes a possible issue with XACE. If the client had permission to
write on the client but not on the window, the previous approach would have
failed.
All the rest of XI uses rc and returns rc in case of error, so make
mpx-related stuff comply. This stops the rest of XI sending the error
manually.
This is just a cosmetic change to be in line with the rest.
We need this for clients that need to set the ClientPointer but don't have a
window on display yet. If used, it will set the device as the ClientPointer
for the requesting client.
