Texture buffers are placed in the same descriptor set with the rest of
the textures.  However, the different code paths that handle textures
have special cases for texture buffers as they create a different
descriptor type (texel buffer instead of combined image sampler).  Image
view serials are used to track the buffer view serials as well so the
texture descriptor cache can handle texture buffers as well.

This CL is missing storage texel buffer support.

Bug: angleproject:3573
Change-Id: Iff80ca22ff9b9957a0c9a3c7aaada1fa54b24ec8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2532653
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>