vello_hybrid: panics on fully transparent colors ("Color fields with 0 alpha are reserved for clipping")

`vello_hybrid` panics on the first frame containing a fully transparent paint: ``` panicked at vello_hybrid-0.0.9/src/schedule.rs:1823: Color fields with 0 alpha are reserved for clipping ``` Fully transparent colors are legal and routine in CSS content — Tailwind apps emit them constantly (`background-color: rgba(0,0,0,0)` as hover-transition base states, `transparent` gradient stops). Hit via blitz/Dioxus Native, where vello_hybrid is the default renderer: a real-world app aborts on its very first frame. Could the scheduler skip zero-alpha draws (they're no-ops visually) instead of asserting?