Cleanup VR Application API with and consolidate with RenderTaskNew

[haraldsteinlechner]

currently [GL|Vk]VRApplicationLayered is inconsistent in terms of setting RenderTask. Most severely IRenderTask interface implementaitons sometimes void RenderTask setters, sometimes not, aardvark.VR mostly uses RenderCommands - for somewhat good reason - OpenGlVRApplicationLayered should support this as well just like VulkanVRApplicationLayered. This allows unification. When renderTaskNew is stable and 5.1 it is the right time to approach this one.