Enhance copyTexImage2D / copyTexSubImage2D tests

[kenrussell]

A bug reported by the Three.js developers against WebKit:
https://bugs.webkit.org/show_bug.cgi?id=222790

demonstrated that the tests of copyTexImage2D and copyTexSubImage2D are likely incomplete. The failure that was reported was only caught by WebKit-specific tests.

The associated tests in the WebGL conformance suite - likely conformance/textures/misc/copy-tex-image-and-sub-image-2d.html - should be updated.

[kimmok]

I think the test is good and should have caught the problem, it's a webkit test infra bug that this wasn't caught.
Can be closed.

[kenrussell]

OK, thanks for your feedback. I re-reviewed https://bugs.webkit.org/show_bug.cgi?id=222790 and it's not 100% clear to me that all of the cases in the revised test are covered by sdk/tests/conformance/textures/misc/copy-tex-image-and-sub-image-2d.html , but will take your word for it.