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/+/1854895Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Cody Northrop <cnorthrop@google.com>