Bump third_party/imgui/repo from `03298fe` to `240ab58` #1227

Job	Run time
Build on Darwin (clang, python_lib, simulated)	1h 1m 34s
Build on Linux (fake, gcc_release, clang, simulated)	1h 22m 51s
Build on Linux (coverage)	10m 3s
Build on Linux (gcc_debug)	47m 2s
Build on Linux (python_lib)	18m 56s
Build on Linux (python lighting-app)	6m 30s
	3h 46m 56s

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