Bump third_party/imgui/repo from `3c435c0` to `605c8d7` #2216

Job	Run time
Build on Darwin (clang, python_lib, simulated)	1h 24m 59s
Build on Linux (fake, gcc_release, clang, simulated)	2h 5m 47s
Build on Linux (coverage)	15m 7s
Build on Linux (gcc_debug)	1h 4m 43s
Build on Linux (python_lib)	25m 5s
Build on Linux (python lighting-app)	7m 3s
	5h 22m 44s

Provide feedback