Fix CopyTexture to support GL_LUMINANCE/ALPHA/LUMINANCE_ALPHA A few WebGL tests were failing on Linux/NVIDIA/passthrough because the luminance workaround doesn't handle the case where the destination texture is a luminance texture. For now, fallback to CPU readback in this case. Bug: chromium:773861 Change-Id: I1b85edccc2e257a5fa19cd496d86443ba96e07ad Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1695922 Commit-Queue: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Geoff Lang <geofflang@chromium.org>