Add support for directly waiting on a proxy timeline. #590

Job	Run time
Build windows-latest \|\| SHARED=OFF \|\| PLATFORM=SDL	20m 45s
Build windows-latest \|\| SHARED=OFF \|\| PLATFORM=libretro	15m 43s
Build windows-latest \|\| SHARED=ON \|\| PLATFORM=SDL	20m 9s
Build windows-latest \|\| SHARED=ON \|\| PLATFORM=libretro	14m 55s
Build ubuntu-latest \|\| SHARED=OFF \|\| PLATFORM=SDL	19m 13s
Build ubuntu-latest \|\| SHARED=OFF \|\| PLATFORM=libretro	18m 58s
Build ubuntu-latest \|\| SHARED=ON \|\| PLATFORM=SDL	19m 36s
Build ubuntu-latest \|\| SHARED=ON \|\| PLATFORM=libretro	17m 50s
	2h 27m 9s

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