When a target texture is created from an EGL image, respecification
of the texture needs to take into account the fact that it din't own
the ImageHelper. Refactor copy and stage code to account for this
possibility as well.

Bug: angleproject:3756
Bug: angleproject:5281
Test: ImageTest.Source2DTarget2DTargetTextureRespecify*
Change-Id: I2e3bd5d1d64e85da521a841423cfe24673efe88f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2524703
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>