Fix texture swizzle cache initialization Texture swizzle cache needs to be initialized so that the keys do not represent a valid swizzle state before any swizzled textures have been cached. BUG=angleproject:1095 TEST=dEQP-GLES3.functional.texture.swizzle.* (all pass) Change-Id: I66c10ed49134875d29b07852488a41a776e8fb67 Reviewed-on: https://chromium-review.googlesource.com/326971 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>