vello_hybrid Spatiotemporal compositing tracking issues

[ajakubowicz-canva]

Context

Adding spatiotemporal compositing to vello_hybrid is a large change, so there are improvements that need followup separate from the initial landing PR.

This issue tracks these improvements.

Tasks

• PushBuf does not need to unconditionally resolve an invalid temporary slot. (comment). Only if a blend will occur. This will require another annotated command.
• Remove allocations from the scheduler - specifically consider removing the duplicated annotated commands. (comment) - vello_hybrid: remove suspending work from scheduler #1174
• Enable native webgl backend
• Fix deadlocking case - requires potentially tracking and reserving slots in advance such that a pending tile can unconditionally complete. - vello_hybrid: remove suspending work from scheduler #1174
• Add mix.
• Some kind of scheduler debug visualizer. (comment)
• Refactor some repeated code. (comment) (comment)