Use facingMode of environment to use rear-facing camera

[jywarren]

spectral-workbench.js/examples/capture/getUserMedia.js

Line 49 in 8d0526d

facingMode: { ideal: "environment" },

this line didn't work on Android Chrome, in #173, but maybe it will on iOS. But in any case let's try variants to get this correctly using the rear-facing camera when possible, by default:

https://developer.mozilla.org/en-US/docs/Web/API/MediaTrackConstraints/facingMode

[jywarren]

https://developer.mozilla.org/en-US/docs/Web/API/Media_Streams_API/Constraints#Example_Constraint_exerciser some more on this