Bump third_party/imgui/repo from `3c435c0` to `cb16568` #900

Job	Run time
Build on Darwin (clang, python_lib, simulated)	1h 54m 58s
Build on Linux (fake, gcc_release, clang, simulated)	2h 12m 34s
Build on Linux (gcc_debug)	1h 9m 53s
Build on Linux (python_lib)	31m 4s
Build on Linux (python lighting-app)	11m 37s
Build on Linux (coverage)	17m 47s
	6h 17m 53s

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