If a buffer is created before a heap in MVKDeviceMemory, the buffer would
be used in some places (for example `MVKDeviceMemory::map` and the heap
would be used in other places (for example `MVKBuffer::getMTLBuffer()`).
This leads to inconsistent memory mappings.

Specifically, there are two cases when this happens:
1. When `vkAllocateMemory` is called with an imported buffer.
2. When `vkAllocateMemory` expects `HANDLE_TYPE_MTLBUFFER` to be exported.

In either case, `ensureMTLBuffer()` gets called before `ensureMTLHeap()`
which sets `_mtlBuffer` to a buffer not allocated from the placement heap.

This change fixes this by making sure that if a placement heap is needed,
we will always create it prior to allocating the buffer. In cases where
the heap is not used at all (imported buffer), we will return error if the
user tries to specify a `HANDLE_TYPE_MTLHEAP` for export.

`_mtlTexture` can still be nil at this point which means that we may
have a memoryless texture attempt to get added to the residency set.

There is a special use case for when importing/exporting a semaphore of
handle type `VK_EXTERNAL_SEMAPHORE_HANDLE_TYPE_SYNC_FD_BIT` and FD == -1.
According to the Vulkan specs, calling `vkImportSemaphoreFdKHR` has the
effect of signaling the binary semaphore and calling `vkGetSemaphoreFdKHR`
has the effect of waiting on the binary semaphore.

Outside of KHR_external_semaphore_fd extension, there is no other way to
achieve this behaviour. For example, `vkWaitSemaphores` does not operate
on a binary semaphore and there is no other defined way for a CPU to send
a signal operation.

In theory, we can use EXT_metal_objects and export the VkSemaphore to a
`MTLSharedEvent` but we do not have access to the `MVKSemaphoreMTLEvent` in
order to determine the `_mtlEventValue` (last encoded wait value).

This functionality is required to implement Venus on macOS which uses these
functions to synchronize the guest semaphore state to the host.

Fix incorrect variable usage in `MVKImagePlane::getMTLTexture()`

Introduce KHR_external_semaphore_fd only for FD==-1

Fix buffer and heap out-of-sync in initExternalMemory()

- Update to latest SPIRV-Cross.
- Add top CMake file to Xcode projects to support editing versions in future.
- Update documentation.

Update version to 1.4.2.