[PATCH] [RECOMMEND 21.4] Stopping timeouts during GC causes lossage on all GUI platforms (X11, GTK and Win32)

G'day. I was experiencing a semi-regular failure of an error checking XEmacs while reading items with Gnus that I tracked down to being caused by animated images. To rotate the animation, XEmacs registers a timeout for them and, in the image finalize method, cancels the timeout. Unfortunately, the method that cancels the timeout will synchronously free a wide range of Lisp objects, which causes heartburn for the GC system -- an assert and, presumably, the risk of a genuine crash without it. This crash is common to all platforms, as they all use the same common code to actually cancel the image timeout. The solution I have implemented in this patch is to modify the common method. Now, rather than immediately cancel the timeout, that common code will register a suitable post-GC action to cancel the timeout once we are safely outside the danger zone. This keeps the change to a single file, and avoids needing to embed more tricky timing logic into the three GUI branches that would then need to be kept in sync. This is recommended for 21.5 as well as HEAD because the same problem is present on both sides. Regards, Daniel