WebCanvas: Handle WebGL context loss

[QuantumBadger]

In the upcoming WebGL support, we don't handle the situation where the WebGL context is lost and restored. While it might be best to detect this externally and restart the app, we could handle this internally by saving all the textures in RAM and re-uploading them when the context is restored, as well as re-initialising all the shaders/etc.

[Revertron]

Is it too complex to be configurable?

[QuantumBadger]

Is it too complex to be configurable?

It'll probably have to be configurable (and off by default) if it's added after the initial release, to avoid conflicting with any user-provided handling.