xwayland: Do not discard frame callbacks on allow commits

Currently, when a X11 client (usually the X11 window manager from a Wayland compositor) changes the value of the X11 property _XWAYLAND_ALLOW_COMMITS from false to true, all pending frame callbacks on the window are discarded so that the commit occurs immediately.

Weston uses that mechanism to prevent the content of the window from showing before it's ready when mapping the window initially, but discarding the pending frame callbacks has no effect on the initial mapping of the X11 window since at that point there cannot be any frame callback on a surface which hasn't been committed yet anyway.

However, discarding pending frame callbacks can be problematic if we were to use the same _XWAYLAND_ALLOW_COMMITS mechanism to prevent damages to be posted before the X11 toplevel is updated completely (including the window decorations from the X11 window manager).

Remove the portion of code discarding the pending frame callback, Xwayland should always wait for a pending frame callback if there's one before posting new damages.

See also:

wayland/weston!293 (merged) !316 (merged)

Edited by Olivier Fourdan

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