Merge ref_vk into main.

[Rykita]

As the title suggests, I've been left to wonder why is the Vulkan renderer a separate repo? It's an outlier in this regard as GL1.4, GLES, GL3.2, Software are all in this repo. Is it due to the experimental nature of the VK renderer?

[protocultor]

Read the whole history of the VK renderer here:
#505 (comment)

[0lvin]

We are always appreciate contributions to ref_vk and when code will have same level of code quality and features set as gl3, it could be merged back to main repository or remove experimental flag at least.

As example of feature set difference:

• Exclusive fullscreen and adaptive V-Sync only with in-game V-Sync on ref_vk#6
• Port the GL3 lighting calculations to Vulkan ref_vk#1

[Yamagi]

The biggest problem is that Vulkans requirements are incompatible with Quake IIs architecture. For example Vulkan can abort a frame after each and every API call, Quake II only has very limited handling for aborted frames. While Vulkan may work well enough for the average use case, there are a lot of corner cases caused by these architectural limitations we weren't able to solve with the time we could invest into it. Since GL3 is is arguably a better renderer feature wise we have given up on Vulkan and moved it into an external repository for user who can't run GL3 but Vulkan.