Vulkan: Support binding texture levels as a rendertarget This CL refactors how TextureVk handles rendertargets. It removes the single rendertarget that previously supported 2D, and expands the layer/level list of rendertargets to handle all cases. Bug: angleproject:3184 Bug: angleproject:3996 Test: Texture2DTestES3.FramebufferTextureChangingBaselevel/ES3_Vulkan Test: FramebufferRenderMipmapTest.RenderToMipmap/ES2_Vulkan Test: FramebufferRenderMipmapTest.RenderToMipmap/ES3_Vulkan Test: ComputeShaderTest.ImageStoreMipmapSlice/ES3_1_Vulkan Change-Id: I466d0389cc6744994f88c40cc388fca694b53a99 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1854895 Reviewed-by: Ian Elliott <ianelliott@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Cody Northrop <cnorthrop@google.com>