Make the EGL surface class abstract.

Make the EGL surface class abstract.

gl::Surface is now the pure abstract interface for egl::Surface, which
can be used by libGLESv2 without requiring typeinfo.

Bug chromium:732667
Bug swiftshader:31

Change-Id: I7d8a5892c5b6186541f84c3cf39e72ac1d6c613d
Reviewed-on: https://swiftshader-review.googlesource.com/10129
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>