egl: Replace IsBound by a pointer to the binding.

egl: Replace IsBound by a pointer to the binding.

IsBound tells if a context or surface is current.  What it does not tell
is, to which thread a context is current, or to which context a surface
is current.  This commit replaces IsBound by a pointer to the binding
thread or context.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>