Move lod-sampling shaders to shader utils Will be reused in a test in a following change. Bug: angleproject:2914 Change-Id: I4e255d5c762f2a6c064b06c558519d82ec6ead5f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2239085 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com>