[sdl] Fix "unresponsible application" issue. Contributes MER#1934
Polling without wl_display_flush() call never catch ping requests. In that case ping-pong works only on other events, such as user input or on frame swapped.
The build is available at https://build.merproject.org/project/show/home:Kaffeine:mer (should work on SFOS-126.96.36.199+)
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