vulkan: add support for importing Metal textures and IOSurfaces

One FIXME: we need to detect when PL_HANDLE_MTL_TEX and PL_HANDLE_IOSURFACE are appropriate, somehow (they only work on sufficiently new MoltenVK, which isn't in a release branch yet; build from trunk to test).

Hoping for some discussion on the new pl_handle_types, which are mapped directly as VkImages rather than as memory.

Also includes my fix for #206 (closed).