Use gl::Object to reference count egl::Image.

Bug 26851951

Change-Id: I5b594bdd34e6fa1074d5439ee2c09f6731839dee
Reviewed-on: https://swiftshader-review.googlesource.com/4534
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>