Bump third_party/imgui/repo from `3c435c0` to `797101a` #2347

Job	Run time
Build on Darwin (clang, python_lib, simulated)	2h 12m 32s
Build on Linux (fake, gcc_release, clang, simulated)	1h 56m 42s
Build on Linux (coverage)	13m 23s
Build on Linux (python_lib)	22m 37s
Build on Linux (gcc_debug)	56m 15s
Build on Linux (python lighting-app)	5m 21s
	5h 46m 50s

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