Bump third_party/imgui/repo from `03298fe` to `7f9533b` #1240

Job	Run time
Build on Darwin (clang, python_lib, simulated)	1m 50s
Build on Linux (fake, gcc_release, clang, simulated)	1h 24m 33s
Build on Linux (coverage)	3m 26s
Build on Linux (gcc_debug)	1m 52s
Build on Linux (python_lib)	3s
Build on Linux (python lighting-app)	1m 26s
	1h 33m 10s

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