Add C API for Vulkan wgpu rasterizer

[afishhh]

Adds a functional C API for creating a rasterize::wgpu::Rasterizer from handles obtained via the Vulkan API, along with the ability to query what extensions/features the rasterizer requires.

This does indeed work which is demonstrated in the new examples/c_glfw_vulkan example which uses GLFW to create a window, manually creates all required Vulkan objects and finally an sbr_vk_rasterizer.

TODO:

• Add some documentation
• There is actually one change that was necessary in wgpu-hal to make this work: https://github.com/afishhh/wgpu/tree/public_physical_device_features, the PhysicalDeviceFeatures structure is required to be able to tell what device features and extensions are necessary but it is private even though it's returned in a public function! I assume this is a mistake and will make an issue/PR to wgpu soon™.
• Implement synchronization in the example.
• Add functions for destroying the sbr_vk_* objects since currently you're forced to leak them.
• Figure out how to handle the case when the library is compiled without the wgpu feature. Should these functions be stubbed out? Is there another way of doing things without making using the API significantly more annoying?

[afishhh]

This is on-hold pending a GPU-acceleration rework in subrandr. The API will probably be changed as I think this may not actually be what we actually want (need to learn Vulkan to know :)).