Add support for Vulkan video extensions

[adam-sawicki-a]

This is a note to myself and a call for feedback on adding support for Vulkan video - a group of extensions, as described here, for example:
https://www.khronos.org/blog/khronos-finalizes-vulkan-video-extensions-for-accelerated-h.264-and-h.265-decode
Extensions VK_KHR_video_queue, VK_KHR_video_decode_queue etc.

One thing to add will be the creation of VkVideoSessionKHR together with allocation and binding of memory required by it, similarly to the existing vmaCreateBuffer and vmaCreateImage. The new function should:

1. Call vkCreateVideoSessionKHR to create VkVideoSessionKHR object.
2. Call vkGetVideoSessionMemoryRequirementsKHR to query for memory requirements.
3. Allocate required memory.
4. Call vkBindVideoSessionMemoryKHR to bind this memory with this session object.
5. Return the VkVideoSessionKHR object and an array of VmaAllocation objects.

The function could look somewhat like this:

VkResult vmaCreateVideoSession(VmaAllocator allocator,
    const VkVideoSessionCreateInfoKHR* pSessionCreateInfo,
    VmaAllocationCreateInfo* pAllocCreateInfo,
    VkVideoSessionKHR* pVideoSession, // [out]
    VmaAllocation** ppAllocations, // [out]
    VmaAllocationInfo** ppAllocationInfos); // [out, optional]

One difficulty is that this object requires a whole sequence of memory allocations of unknown length, not just one. This length is not known before the session object is created. Questions:

1. How to return this array of allocations of unknown size? One solution would be to only support a function like vmaAllocateMemoryForBuffer, so that users need to create the session object themselves, query for memory requirements to know the length, and then allocate memory using VMA.
2. How to interpret a single VmaAllocationCreateInfo structure if the session object might require multiple allocations from different memory types (e.g. one from GPU and one CPU memory)? Should the function accept only some new, simplified structure that doesn't offer manual control over memory types?