with-blending is disabling blending at end of scope rather than restoring

[cbaggers]

Was meant to have been fixed by https://github.com/cbaggers/cepl/issues/94 but looks like I missed it. Bah.

Well we know the drill.

• refresh ourselves with how blending works and how it interacts with attachments (per attachment blending)
• Add vars to the context to track this info
• modify scoped macro to handle restore