EGL: Fix a bug in ImageTest
EGL: Fix a bug in ImageTest

During cleanup, we were incorrectly calling
glDeleteRenderbuffers on a texture object.
Also update comments to correctly reflect intent.

Bug: angleproject:5018
Change-Id: I28eaf6a23056f3b09eebb7331620e6a27cb14302
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2406614
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Commit-Queue: Mohan Maiya <fbcab303f71178634c736986d06722fc897795f2@samsung.com>

ppy/angle,ppy/angle,ppy/angle,ppy/angle