Sampling from rectangle textures produces incorrect results for glCopyTextureCHROMIUM.
Temporarily disable this path while the underlying issues are investigated.

BUG=990368

Change-Id: I690252ed4d09cdb6a7fd94f6f24d1300c53afc1b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1832496Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>