ES3 requires that only the red channel has data when sampling from depth
textures while desktop GL treats them as luminance textures.  Set the
swizzle state of the texture to sample 0 from the blue and green channels
in TextureGL.

Passes 75 new tests in dEQP-GLES3.functional.texture.format and
dEQP-GLES3.functional.texture.specification.

BUG=angleproject:884

Change-Id: I863472961dcbbf8af864f5e1d85a4aea41f47bda
Reviewed-on: https://chromium-review.googlesource.com/298481Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>