pub const SYNC_PRESENT: u32 = _; // 1u32Expand description
[TensorDesc::sync] carries a real handle. When clear, sync is
meaningless and must not be waited on.
A flag rather than a sentinel because every plausible sentinel is a
legal handle somewhere: 0 is a valid GLsync-adjacent value and a
valid fd number is never negative, so -1 and 0 cannot both be
spare across the kinds this field spans.
Under [kind::D3D11_TEXTURE] sync is not a handle at all: it is a
value on the timeline of the fence whose NT handle
[TensorDesc::ptr] carries, so a producer that sets this flag must
also fill in ptr.