getCameraCssVars

The camera, as custom properties.

Separate from the colours because these change on every zoom rather than on a theme swap, and because they are the two variables an app most often needs in CSS: anything drawn in the DOM over a zooming canvas has to divide by the zoom to keep a constant on-screen size. @mocanvas/compat maps tldraw's --tl-zoom and --tl-scale onto them; without them every calc() in a migrated stylesheet resolved to nothing and took its whole declaration with it.