Bump third_party/imgui/repo from `001f102` to `99913b5` #905

Job	Run time
Build on Linux (gcc_debug)	1h 1m 34s
Build on Darwin (clang, python_lib, simulated)	1h 32m 33s
Build on Linux (fake, gcc_release, clang, simulated)	2h 38m 50s
Build on Linux (coverage)	14m 45s
Build on Linux (python_lib)	35m 20s
	6h 3m 2s

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