When a pic needs dynamic vertices (eg, for sub-pics), a descriptor set
is allocated and updated if one has not been created for the pic. This
is done each frame: the descriptor sets are recycled (there currently is
rarely a need for more than a small handful of dynamic descriptors, so
64 should be plenty for now).
Unfortunately, due to the order of operations issue between draw items
getting queued and submitting commands to vulkan (the cause of the pics
not rendering correctly per 8fff71ed4b),
the validation layers complain (correctly) about the command buffers
being executed with updated descriptor sets. Getting the canvas system
up and running will fix that.
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