Implement accessing textures through an index. It uses the same
interface as OpenGL, the main difference is that Vulkan bindings are
forced to be arrayed (the binding index doesn't change for stacked
textures in SPIR-V).
Layered framebuffer attachments is a feature that allows applications to
write attach layered textures to a single attachment. What layer the
fragments are written to is decided from the shader using gl_Layer.