Add a SPA_PARAM_BUFFERS_metaType in the Buffers object. This contains a
bitmask of the mandatory metadata items that should be included on a
buffer when using this Buffers param.
Make the buffer allocation logic skip over the Buffers params that
require unavailable metadata.
This can be used to, for example, enforce specific metadata to describe
extra buffer memory (such as the meaning of generic file descriptors).
One such use is the explicit sync, where an extra buffer data is needed
for the sync fd along with metadata that contains the sync_point.
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