Ensure we have a GL context before deleting View layers

Bug #7391098

The existing code was doing a make current to guarantee we have a
current context. This can however fail when the surface is destroyed
which could lead to GL calls without an EGL context, and therefore
potential leaks. This change fixes the issue by using a technique
found in HardwareRenderer.destroyHardwareResources(). If the surface
is not available then we use a special 1x1 pbuffer as a way to get
a valid context.

Change-Id: I716d3799bf90120d793d76e90a83956837ecd491