Bump third_party/imgui/repo from `2860d7b` to `f5befd2` #1680

Job	Run time
Build on Darwin (clang, simulated)	39m 5s
Build on Linux (python lighting-app)	4m 43s
Build on Linux (gcc_debug)	54m 11s
Build on Linux (python_lib)	14m 25s
Build on Linux (coverage)	18m 39s
Build on Linux (fake, gcc_release, clang, simulated)	49m 1s
	3h 0m 4s

Provide feedback