In GLES 3.0, depth textures where changed to behave like RED textures,
but in GLES 2.0 with ARB/OES_depth_texture, they were treated as
luminance textures.
This change produces the expected behavior on GLES 2.0 for the GL
backend and on GLES 3.0 for the Vulkan backend.

Bug: angleproject:3540
Change-Id: Iad6b4e03cf947b27eb97dbb10419bc8bcdb11024
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1666363Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: James Dong <dongja@google.com>