Keep state variable to enumerate things. This allows us to remove the
expectation that the same index will give the same item.
Use a new rectangle type for video size. When enumerating sizes, this
allows us to specify relations between width and height more easily.
Remove the struct and bytes type, they are like pointer.
Add filter to enum_formats to speed up enumerations.
Add generic pointer metadata type that is copied on import
Pass array of pointers to SpaBuffers in use_buffers.
Make sure we use the same data and metadata as the imported buffer.