Do not execute in background

[daleeidd]

In certain cases memory would slowly increase indefinitely. It would be cleared when editor became active again but would like to avoid leaks altogether.

This could be a Unity bug but more effective to patch than report. It also has the benefit of saving energy.

[mstevenson]

Thanks for this, I noticed that the Editor would crash after sitting idle in the background for a few hours with an ocean visible, and I suspect that this was the cause.