Add attribute to mark pNext pointers with the Vulkan struct they extend

[Rob2309]

This PR adds an attribute VMA_EXTENDS_VK_STRUCT similar to VMA_LEN_IF_NOT_NULL that allows detecting programatically which Vulkan structure will be extended with a given pNext argument.

For example:

VMA_CALL_PRE VkResult VMA_CALL_POST vmaBindBufferMemory2(
    VmaAllocator VMA_NOT_NULL allocator,
    VmaAllocation VMA_NOT_NULL allocation,
    VkDeviceSize allocationLocalOffset,
    VkBuffer VMA_NOT_NULL_NON_DISPATCHABLE buffer,
    const void* VMA_NULLABLE VMA_EXTENDS_VK_STRUCT(VkBindBufferMemoryInfoKHR) pNext);

with this attribute, generators can detect and enforce that pNext points to a structure that is allowed to be present in the pNext chain of VkBindBufferMemoryInfoKHR.

[adam-sawicki-a]

How is it going to be used? Can you please show an example of how it is used and why it is needed?

[Rob2309]

How is it going to be used? Can you please show an example of how it is used and why it is needed?

I would take the ash rust bindings as an example.
In ash, there is a trait called ExtendsBindBufferMemoryInfo that is only implemented for structs that are allowed to be included in the pNext chain of VkBindBufferMemoryInfo as per the Vulkan specification.

With the attribute introduced above, a code generator could automatically generate a typesafe wrapper for vmaBindBufferMemory2 that only allows passing an allowed structure as pNext.

This could probably be somehow extracted from the documentation comment, but I think parsing a custom attribute is cleaner and simpler.

[adam-sawicki-a]

OK, thank you for the explanation. I accepted and merged your change without any modifications. I hope it will be useful.