When destroying a surface, only unmake current if necessary

Bug #9404946

The Calendar app destroys one of its popup windows during a draw
traversal. Without this patch, we end up with no current context
nor surface after we're done gathering display lists. This means
that all of our EGL/GL calls will either fail or have undefined
behaviors. This could explain the PBO crash we are seeing with
the monkeys. Without a proper GL context, the driver returns NULL
when we map the PBO in CPU memory.

Change-Id: I210cf724be73da909a7621f807298a9f4a58e61d