Non-zero layer attachments are FBO attachments of 3D textures bound
from a layer other than zero. These haven't ever worked AFAIK. Fix
them by retrieving the correct layer from the FBO attachment.

Note: 3D attachments are still broken with PBO ReadPixels. The fix
for those will come in a subsequent patch.

BUG=angleproject:1290

Change-Id: I5417e7374188dd320e1209d006723ce070f98561
Reviewed-on: https://chromium-review.googlesource.com/323472
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>