First working version of TpvGUIVulkanCanvas in the PasVulkan GUI subframework, it works completely without render-to-texture tricks, so it is rendered directly to the final framebuffer, but which is a bit tricky with the two-pass front-to-back-opaque and back-to-front transparent GUI rendering strategy, so it is a three-pass GUI rendering strategy here, with a third back-to-front opaque VulkanCanvas render pass as "first" pass.