Feedback for over/under exposed pixels in Live View

[Olliebrown]

As part of showing the live-view stream, we should provide a toggle to activate an overlay of which pixels are at 100% or 0% indicating an over or under-exposed image respectively. For efficiency, this should be implemented in a fragment shader as iterating over all the pixels in JavaScript would be very slow. This would likely require changing the entire live-view widget to use a WebGL context instead of a canvas 2d context like it currently does.