Investigate SwiftShader as a software Vulkan implementation for CI purposes

SwiftShader has been adding Vulkan support in addition to OpenGL (ES). Investigate if this is something we could build ourselves and deploy in some CI configurations to allow running autotests on Vulkan.

https://swiftshader.googlesource.com/SwiftShader

Status 28/10/2019: I managed to produce a Vulkan library with MSVC 2019 from SwiftShader HEAD. Combined with setting VK_ICD_FILENAMES this allows running an application with the SwiftShader pure software Vulkan implementation. Out of the QRhi manual tests the ones that do not use textures seem to work, which is pretty impressive, although with limited performance. The ones that use textures either crash or hang however so this needs some further investigation.