Set up the Vulkan version of the glTF Demo capable of rendering with GLSL shaders generated by Metashade

[Pavlo Penenko]

Metashade can't generate GLSL yet, but we need this C++ testbed first. Initially, I'm going to bring up the Vulkan version with GLSL shaders cross-compiled from HLSL shaders generated by Metashade (see #41 and so forth). The override mechanism will mirror the one I have in place for HLSL. When cross-compiled shaders work, GLSL generation work can begin.