Update copy validation regarding ext textures * Updated the validation functions for the following function to accept TEXTURE_EXTERNAL_OES: glCopyImageSubDataEXT() * Updated unit test CopyTextureTestES3.CopyImageSubDataToEGLImage * Using TEXTURE_EXTERNAL_OES as target for glFramebufferTexture2D() requires EXT_YUV_target. Therefore, the texture related to it is used instead. Bug: angleproject:361372990 Change-Id: I905d4cf89522ae3158516b42b15705d1044c6e97 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5940404 Reviewed-by: Charlie Lao <cclao@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Amirali Abdolrashidi <abdolrashidi@google.com>