rx::mtl::ColorBlitUtils: Use a hash map for shaders We're going to be adding another parameter here (linear-to-sRGB conversion). Prepare the ground by moving this to a hash map. See the similar operation in https://crrev.com/443362. Bug: angleproject:7907 Change-Id: I33f815c55f209927fc7c24e5d8459e337f70cfb4 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4847067 Commit-Queue: ccameron chromium <ccameron@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>