AppVeyor doesn't know how to build Vulkan

[mcclure]

Therefore all PRs against dev are currently failing, eg https://ci.appveyor.com/project/bjornbytes/lovr/builds/43991150 for #556

[bjornbytes]

Previously I've tried installing the Vulkan SDK in the appveyor script, but it timed out. Since LÖVR only uses the headers, maybe KhronosGroup/Vulkan-Headers could be cloned instead (and CMake could be tricked into using that, or there could be an option to specify the Vulkan include path instead of using FindVulkan). For glslangValidator, the standalone binary could be built from deps/glslang. This would mean building glslang twice -- once for lovr and once for the standalone exe, unless CMake enables ENABLE_GLSLANG_BINARIES.

[bjornbytes]

appveyor can build the dev branch now.

[mcclure]

Cool