Vulkan Update 2 todo

[floooh]

• add tests for _sg_track_t funcs
• Windows support
• consider placing stream-update buffers in host-visible memory, make them double-buffered and don't update those buffers via staging
• implement streamlined barrier system (do a similar 'time-warp' of recording barriers into their own command buffers, but only inside render passes - also see the barrier system section in the vulkan blog post)
• expose memory allocator interface and proof-of-concept with VMA
• related: consider using separate command buffers for compute commands and offscreen passes? Those wouldn't need to wait for the present-complete-semaphore!
• support multiple swapchain passes (need to handle multiple present-complete-semaphores)
• implement object labels (only makes sense once RenderDoc actually works)
• port to NVIDIA and find optimal heap types for uniform- and descriptor-buffers
• native resource injection and sg_vulkan_*_info objects and functions