Reduce GdiEngine input latency

[lhecker]

This commit reduces GdiEngine's average display latency by 8ms,
which caused it to miss a v-blank about half the time at 60Hz.

Closes #15607

Validation Steps Performed

Input latency with frarees/typometer matches conhost from Win10 ✅

[DHowett]

Well, I can imagine a thousand tiny ways in which this will break stuff... but let's go for it!

Will this change the teardown behavior?

[lhecker]

It'll only change the way _hPaintCompletedEvent works, because it might wait on an DWM event after having told our OneCore code that we're done rendering already. I don't see how waiting on an event might break anything.
And on the topic of OneCoreUAP I don't understand the code comments in the first place. RenderThread::WaitForPaintCompletionAndDisable talks about our renderer acquiring the display? I don't get it. Are you saying that we can get exclusive display ownership on OneCoreUAP? That'd be a fun step back to the 90s hahaha. Application is stuck/hung --> "Guess you can't alt-tab anymore. lol"