Spec: Addressing comments

[FrankGalligan]

• Initialized last_vert_added
• Normalized usage of ans_decoder_
• Removed parsing of num_new_vertices
• Removed num_corners
• Added prediction_rans_data_buffer
• Fixed spelling of orientations
• Replaced SubtractInt64Vectors with SubtractVectors
• Initialized pred_vals
• Changed point_to_corner_map from array to counter
• Addresses comments in Comment period for Draco extension KhronosGroup/glTF#1114

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If your company signed a CLA, they designated a Point of Contact who decides which employees are authorized to participate. You may need to contact the Point of Contact for your company and ask to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the project maintainer to go/cla#troubleshoot.
• In order to pass this check, please resolve this problem and have the pull request author add another comment and the bot will run again.