Fake transfers for getMappedRange-created ArrayBuffers? #2072

Originally posted by @kainino0x in #747 (comment)

• Non-transferable ArrayBuffers (mapped ArrayBuffers shouldn't be transferable to other threads). (We can do this without JS changes by faking transfers, by making them actually copies instead.)

We might be able to make this work entirely within our spec instead of having to go change the behavior of ArrayBuffer. In more detail:

If you tried to transfer a getMappedRange-created ArrayBuffer, it would appear to work entirely normally - old ArrayBuffer would get detached - except the underlying data would get copied into a new, run-of-the-mill ArrayBuffer, which means that any writes to the new ArrayBuffer would not appear in the GPUBuffer upon unmap().