Examples: Add Win32 Vulkan example.

[jristic]

I'm not sure if you'd want to maintain a Win32/Vulkan example since I expect the SDL3 one is sufficient for most people, but I put together one for my own sake and I figured I may as well share in case you do. I basically took the Win32/DX12 example, removed the DX12 code and then added the Vulkan code from the SDL3/Vulkan example. Really the only new code I wrote was the extensions for surface creation:

ImVector<const char*> extensions;
extensions.push_back("VK_KHR_surface");
extensions.push_back("VK_KHR_win32_surface");
SetupVulkan(extensions);

[ocornut]

Thank you. It looks good so I'll probably eventually merge, but I admit maintaining 4 Vulkan examples is a little bit of a weight.

I'll be tempted to see if for some of the functions shared among those 4 examples we can sneakily move them under the backends are optional helpers.