Bump third_party/imgui/repo from `3c435c0` to `ccb6646` #1248

Job	Run time
Build on Linux (fake, gcc_release, clang, simulated)	2h 0m 34s
Build on Linux (gcc_debug)	1h 2m 36s
Build on Darwin (clang, python_lib, simulated)	2h 7m 58s
Build on Linux (coverage)	15m 54s
Build on Linux (python_lib)	23m 42s
Build on Linux (python lighting-app)	5m 48s
	5h 56m 32s

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