Bump third_party/imgui/repo from `3c435c0` to `d78e823` #2431

Job	Run time
Build on Darwin (clang, simulated)	54m 59s
Build on Linux (fake, gcc_release, clang, simulated)	53m 11s
Build on Linux (coverage)	13m 9s
Build on Linux (gcc_debug)	1h 3m 30s
Build on Linux (python_lib)	23m 20s
Build on Linux (python lighting-app)	6m 31s
	3h 34m 40s

Provide feedback