[Reland] Relocate shaders to own dir

Consolidate all shader impls under src/shaders/.

(reland of https://skia-review.googlesource.com/c/17927/)

Change-Id: I7918bdc1aafe842ed194412ba95b9ae53a2ec1d7
Reviewed-on: https://skia-review.googlesource.com/18146
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
45 files changed