Improve our EGL management when pausing / resuming.

We now release the EGL context when pausing. This
is required to reduce the chance of running out of
contexts on devices which support a limited number
of active EGL contexts.

Someday we may implement a global EGL context
manager that will allow us to let EGL contexts
persist until another activity needs an EGL
context. But for now, without such a manager,
we take the conservative approach.

Separately, we now terminate EGL when
pausing on Sapphire. This is a requirement of the
Sapphire OpenGL driver.