Bump third_party/imgui/repo from `03298fe` to `7f9533b` #348

Job	Run time
Build on Darwin (clang, python_lib, simulated)	55m 34s
Build on Linux (fake, gcc_release, clang, simulated)	1h 23m 22s
Build on Linux (coverage)	9m 42s
Build on Linux (gcc_debug)	46m 35s
Build on Linux (python_lib)	19m 41s
Build on Linux (python lighting-app)	6m 3s
	3h 40m 57s

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