[HTML5] Attempt to estimate device refresh rate at startup

[zshoals]

I'm nooby so maybe this isn't a viable approach (or perhaps it's too naive), but I figured I would try adding this feature rather than not try. Sorry if it doesn't work out.

HTML5 does not have a way to just get the device's refresh rate through some query. It can only be estimated at this time.

This code briefly collects the frametime results for successive requestAnimationFrame calls, and tries its best to make an estimate of the monitor's refresh rate before animate() begins and before the System.start callback executes. This test is run at this point so that (hopefully) nothing else important or computation intensive is happening and we get mostly accurate results.

Caveats:

• Introduces a variable amount of startup delay, dependent on the current refresh rate. I chose 90 frametime samples as a compromise between accuracy and duration of the test since it seemed reasonable (I considered 3 seconds on 30hz as the worst case scenario, 1.5 seconds on 60hz seems passable, etc). I know this isn't great but I don't see a way around it.
• Haven't tested it on toaster ovens. RequestAnimationFrame results there may be more prone to variance caused by computations in the background, which might cause inaccurate results. This may be an issue just generally speaking, but I haven't yet gotten an incorrect refresh rate from my limited tests, however the system I test on is pretty decent.
• Only set at startup once as of now. The refresh rate might possibly change, but I don't think there's a reliable/simple way to know if it actually changed or if something really intensive is occurring due to user code.

[RobDangerous]

Oooh, super cool! Will first fix #1277 so this doesn't confuse me, then merge your pull request, then add the option to run the test while the application is already running (should work fine while on a title screen for example).