Make Vector3 serialisable.

Slight tweak to support using binary serialisation to store lists of positions for distributing groups.
Implement slight hack to Android support which makes it try to use a stencil buffer.

Doesn't solve the problem of MonoGame on Android not setting the PresentationParameters depth format correctly, or paying any attention to the preferred depth format. Just makes the fixed thing it tries to do, be what we want it to do.
Fix typo in previous change to Vector3.
Set PresentationParameters to correspond to what the hardware actually supports, rather than leaving it set to nothing useful. We can use this to determine at runtime whether the depth buffer and stencil buffer are available.