add vkgears demo

Mesa has had Vulkan support for many years now, and so far no Vulkan demos! This needs to change!

So here's a Vulkan port of the gears demo, with a bunch of limitations:

• Only Wayland support. The WSI code is more or less separated out, and it should be possible to support X11 as well. But I haven't done that work yet, and I'm a bit unsure if this is the right approach or not. More below.
• No support for MSAA. It's not hard to add or anything, just some more typing.
• No fullscreen support. Again, not hard to add, just typing.
• No swap-interval support, hard-coded vsync off for now. Again, just typing needed.
• No support for non gamma-correct rendering. There's no reason to not always do sRGB on Vulkan IMO. Lighting in gamma space is not great.
• No support for stereo rendering (e.g multiview). I don't have hardware to test this, and even if I had it would be pretty far down on my list of things to add.
• No keyboard input.

I think the biggest question mark here is if doing manual WSI like I've done for Wayland here is a good idea in the first place. I was kinda following glxgears precedence here, but I'm not sure it's the right thing to do. I could imagine using either SDL or GLFW here would make things a bit easier to maintain... but it'd also pull in some non-trivial dependencies.

I don't really want to create sepatate vkgears_wayland / vkgears_x11 binaries like we do for es2gears, that just seems messy to me. If so, it's be better to do similar to vkcube does, and either pick API automatically, or explicitly based on a command line switch.

Thoughts?

This MR contains a few patches from !92 (merged). Please review those in that MR, because I want to land that one first.