Bump third_party/imgui/repo from `3c435c0` to `08b1496` #2329

Job	Run time
Build on Darwin (clang, python_lib, simulated)	1h 31m 8s
Build on Linux (coverage)	21m 35s
Build on Linux (gcc_debug)	1h 17m 13s
Build on Linux (fake, gcc_release, clang, simulated)	2h 9m 42s
Build on Linux (python_lib)	29m 23s
Build on Linux (python lighting-app)	7m 24s
	5h 56m 25s

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