When the source and target textures are the same texture but different
levels, there is a corner case which makes the framebuffer incomplete on
calling glCopyTexImage2D and fails
conformance2/textures/misc/copy-texture-image-same-texture.html

Bug: angleproject:2994
Change-Id: I906869c78aefded5688d4491e93b95dd2662175c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1697262Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>