This change implements glCopy[Sub]TextureCHROMIUM in GPU.  As with the
previous change implementing glCopyTex[Sub]Image2D, it currently only
selects the shader path if the texture is already defined.

Bug: angleproject:2958
Change-Id: Ia1b5625f92e6c9f91807c9b601e5c34d2d5e5c30
Reviewed-on: https://chromium-review.googlesource.com/c/1392394
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>