Combine es2::Image and es1::Image into egl::Image.

Change-Id: Iadd4ce8ac36ad3458741f244423731fa00d7fd84
Reviewed-on: https://swiftshader-review.googlesource.com/2982
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>