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/326971Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>