Bump third_party/imgui/repo from `001f102` to `1ab63d9` #1091

Job	Run time
Build on Darwin (clang, python_lib, simulated)	1h 6m 6s
Build on Linux (fake, gcc_release, clang, simulated)	2h 10m 1s
Build on Linux (coverage)	17m 0s
Build on Linux (gcc_debug)	1h 1m 16s
Build on Linux (python_lib)	26m 3s
Build on Linux (python lighting-app)	9m 53s
	5h 10m 19s

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