Expand glossary: pixel storage and color volumes

I thought it would be a shame to let swick/wayland-protocols!18 (comment 1270447) be lost in the sea of Gitlab comments, so I wanted to pull those terms into the glossary. While doing so, I reorganizd the glossary, and included the pixel storage terms that I had written in an email to dri-devel last year.

This is just an incremental step.