Before OpenGLES3.0, framebuffer object with unequal sized attachment is
considered incomplete. That restriction has been removed in ARB version
of spec and is legit use in OpenGLES 3.0. This adds a test for this
specific usage.

Bug: b/181800403
Test: FramebufferTest_ES3.AttachmentWithUnequalDimensions
Change-Id: I79608bfd8f10855c6fc47511e080ba3ca4e46c59
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2824366Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Charlie Lao <cclao@google.com>