sampleable property
Whether this may be bound to a sampler at all.
False for a deviceTransient target, which is the closest this API has to
Impeller's tile memory: WebGPU has no transient storage mode, so the
honest translation is an attachment allocated without TEXTURE_BINDING.
Carried rather than derived so a bind of one is a Dart assertion rather
than a browser message about a usage flag.
Implementation
final bool sampleable;