[egl] add support for vendor-specific GL extensions

on implementations which have a single EGL backend, it is straight-
forward to query the EGL implementation for the supported GL extensions

the mechanism added in this commit will only work on implementations
which have a single EGL backend; on implementations which have
multiple backends, this feature should be disabled at compile-time

for EGLImage extensions, call into a wrapper layer implementation to
translate the meta-EGL object into the vendor object before calling into
the vendor's EGLImage implementation

Change-Id: Ie0e9b2dedebc534be9bca7ba0556b75eae9d006f