This extends cogl_onscreen_x11_set_foreign_xid to take a callback to a
function that details the event mask the Cogl requires the application
to select on foreign windows. This is required because Cogl, for
example, needs to track size changes of a window and may also in the
future want other notifications such as map/unmap.
Most applications wont need to use the foreign xwindow apis, but those
that do are required to pass a valid callback and update the event mask
of their window according to Cogl's requirements.
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