Bump third_party/imgui/repo from `03298fe` to `2156db7` #358

Job	Run time
Build on Linux (fake, gcc_release, clang, simulated)	1h 50m 42s
Build on Linux (gcc_debug)	57m 33s
Build on Darwin (clang, python_lib, simulated)	1h 37m 23s
Build on Linux (coverage)	31m 35s
Build on Linux (python_lib)	1h 37m 19s
Build on Linux (python lighting-app)	29m 19s
	7h 3m 51s

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