Philip Langdale authored Dec 14, 2018 and Niklas Haas committed Dec 21, 2018

In prepartion for supporting importing external memory, we need to
define separate caps for import handle types. In theory, a Vulkan
implementation is free to only support a handle type for one
direction and not the other.

This also means we have a field for import caps for `pl_sync`. This is
always set to 0 for now as we don't have a use-case for importing
semaphores.