[Feature request] Add Emscripten support for Sensor API

[foxtacles]

Emscripten exposes the devicemotion event as a callback, so I think it should be possible to add Emscripten as a supported backend for SDL's Sensor API.

It does come with the caveat of requiring explicit user permission beforehand but that's the case for other APIs as well.

[antonegas]

I have been looking into making a PR for this but I have been stuck on one detail. There doesn't seem to be a way to check if accelerometer or gyroscope is available before the first event callback is triggered (at least not one supported by all major browsers).

What would be the expected way to handle this. Should the number of sensors be updated first after Detect is called or is it fine that GetCount gives a different result even if Detect is not called inbetween.

[icculus]

It's always a single rotation and acceleration sensor, right? Unless we have reason to believe otherwise (the layout isn't "standard" or whatever), just lie and say there's one of each, and if there isn't really one, it'll just never send events.